It looked like Amazon.com Inc.’s yearslong quest to build a shopping business in China was a bust in July when it folded a big part of its local business.

In fact, Amazon’s China business is bigger than ever. That is because it has aggressively recruited Chinese manufacturers and merchants to sell to consumers outside the country. And these sellers, in turn, represent a high proportion of problem listings found on the site, according to a Wall Street Journal investigation.

The Journal earlier this year uncovered 10,870 items for sale between May and August that have been declared unsafe by federal agencies, are deceptively labeled, lacked federally-required warnings, or are banned by federal regulators. Amazon said it investigated the items, and some listings were taken down after the Journal’s reporting.

Of 1,934 sellers whose addresses could be determined, 54% were based in China, according to a Journal analysis of data from research firm Marketplace Pulse.

Amazon’s China recruiting is one reason why its platform increasingly resembles an unruly online flea market. A new product listing is uploaded to Amazon from China every 1/50th of a second, according to slides its officials showed a December conference in the industrial port city of Ningbo.

Chinese factories are squeezing profit margins for middlemen who sell on Amazon’s third-party platform. Some U.S. sellers fear the next step will be to cut them out entirely.

Tony Sagar began noticing the China effect around 2015. His company, Down Under Bedding in Mississauga, Ontario, had sold goose-down duvets on Amazon since 2014—these days, for $699 for a queen-size version. Then Chinese competitors hit, listing goose-down duvets for sometimes a sixth his price. He bought one and had it tested: Inside was inexpensive duck down.

The Journal in October bought a duvet from the same Amazon seller claiming “100% Fill With Goose Down” and had it tested. The result matched Mr. Sagar’s: duck feathers.

“They’re claiming they’re selling a $500-$700 duvet based on false specifications, so people say, ‘$120, it’s a good deal!’ ” Mr. Sagar said. “Amazon is making a direct push for these factories in China.”

In response to this article, an Amazon spokesman said, “Bad actors make up a tiny fraction of activity in our store and, like honest sellers, can come from every corner of the world. Regardless of where they are based, we work hard to stop bad actors before they can impact the shopping or selling experience in our store.”

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Mr. Sagar’s discovery came as Amazon was expanding a campaign it started around 2013 urging Chinese businesses to sell directly to consumers abroad. An Amazon sales director, Alicia Liu, at a 2017 conference told Chinese business people she was leading a team in China, drawing on her previous experience cutting out middlemen in Walmart Inc. ’s supply chain.

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“We help factories directly open accounts on Amazon and sell to U.S. consumers directly,” a video shows her telling them. “This is our value.”

A wave of Chinese merchants have joined Amazon’s millions of third-party sellers worldwide, who collectively represent more than half of Amazon’s physical gross merchandise sales.

Among the 10,000 most-reviewed accounts on Amazon’s U.S. site whose locations could be determined in October, about 38% were in China, Marketplace Pulse calculates, compared with 25% three years ago.

The Amazon spokesman said 38% “is a significant exaggeration of the real percentage of the top ten thousand’’ and that the methodology is flawed, citing what it said were problems with the way in which the analysis used seller review counts to estimate the percentage. Marketplace Pulse said it stood by its analysis.

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Amazon’s third-party marketplace, which connects merchants and buyers around the world, is crucial to the company’s growth. At the same time, even though it has become a source of fake or dangerous goods, Amazon has denied it is liable for what’s sold there, saying in court cases that it neither makes nor sells the products in question.

In its annual Securities and Exchange Commission filing this year, Amazon disclosed for the first time that counterfeits and fraudulent products are a risk factor. It said Amazon may be “unable to prevent sellers in our stores or through other stores from selling unlawful, counterfeit, pirated, or stolen goods,” among other issues.

Amazon said it recruits sellers in many countries and that these merchants are central to its goal of offering customers good selection at good prices. Amazon said it requires products to comply with applicable laws and regulations. It said that in 2018 it blocked more than three billion suspect listings for various forms of abuse.

Consumers and businesses with safety and intellectual-property grievances have found it hard to hold Chinese sellers accountable—in part because Amazon doesn’t require its sellers to provide their locations to the public on its U.S. site.

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The Journal identified sellers as being in China from their pages on Amazon’s site in Mexico, where regulations require sellers to list their locations on Amazon—a method Marketplace Pulse also uses.

New sellers from China are hurting merchants that have built Amazon businesses offering products they import from Chinese factories, said Amazon seller Bernie Thompson. His Plugable Technologies in Redmond, Wash., lists electronics products made in China. Since about five years ago, Chinese manufacturers selling on Amazon have priced him out of some product categories, he said—some of them his own suppliers and others who game Amazon’s rating system, he said.

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Amazon seller Zhao Weiming said the site “is the most cost-effective way to sell into the United States.” The Guangzhou businessman experimented several years ago listing gadgets on Amazon before settling on cosmetics and essential oils, he said, establishing factories to produce them under the name Lagunamoon. He said his company earns $50 million a year on Amazon.

Listings for some popular Lagunamoon essential oils claimed they were U.S. Food and Drug Administration approved, until the Journal raised the matter with Amazon and Mr. Zhao in early November. An FDA spokesman said essential oils wouldn’t meet the agency’s definition of an approved product, although it was possible some component—a dye, say—might be approved.

Mr. Zhao said FDA requirements are complex and he didn’t want to use tens of thousands of words to explain.

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Concerns at Amazon about Chinese listings arose several years ago in its China team, which noticed that as local sellers flocked to the platform, it saw increasing patterns of fraud, counterfeits and unsafe products, said former Amazon employees in China.

Washington state’s attorney general’s office said Amazon agreed to pay $700,000 as part of a legally binding agreement after an investigation revealed dozens of products marketed toward children had excessive lead and cadmium. The products were made in China, the office said, some sold by China-based third parties. Amazon didn’t admit wrongdoing.

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Cheap Chinese counterfeits drove Kevin Williams, a Utah seller of water-powered cleaning brushes on Amazon, to lay off six employees this year—most of his U.S. staff, he said. He and his co-founder developed their patented Brush Hero product, made in the U.S. and U.K., in 2015 after finding it difficult to clean their vehicles, selling them on Amazon for $34.99.

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Poorly made copies began appearing in 2018 on Amazon, eventually listing for as low as $9.99, some claiming to be the Brush Hero brand, he said. Buyers, unaware they were fake, trashed Mr. Williams’s products on his Amazon page, he said. When he complained to Amazon, he said, it told him to order the alleged counterfeits and test them. Amazon removed brushes he proved counterfeit, he said, but it could take weeks for them to arrive for testing, and new counterfeits kept popping up.

He dropped prices to $19.99, which “pulled out the rug from us from a cash-flow perspective” he said. A retailer declined to give him a large contract. “He said, ‘What the heck, your Amazon reviews are terrible,’ ” said Mr. Williams, who calls his company “walking dead.”

Amazon said that it acted on infringement cases where Brush Hero provided adequate information and that it has introduced programs for sellers to fight counterfeits, including one called Project Zero that uses automation to scan Amazon stores and remove suspected counterfeits.

Counterfeits and inauthentic reviews “have all gone through the roof with the rise of Chinese sellers,” said Chris McCabe, an investigator for Amazon until 2012, now a consultant helping Amazon sellers counter illicit competition.

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Inauthentic reviews for listings from China can trick Amazon’s algorithm into boosting products, people outside Amazon familiar with the activities said. A search for “travel pillows” in August presented products with names such as MLVOC offered by sellers whose names matched those of Amazon accounts registered in southern China.

The Journal ordered MLVOC-brand pillows from sellers named Corki and Kingstyle Supplies, and got gift cards offering a free pillow if the buyer emailed an address—the same address for both sellers. A “Gift card team” responded, asking the buyer to give a five-star review for which it promised an Amazon gift card. Of one MLVOC pillow’s roughly 2,000 reviews, about 86% have five stars.

Amazon policy forbids making inducements for positive reviews. Amazon said it investigated and took action, eventually reinstating Kingstyle and Corki. Amazon said in some cases it will reinstate seller accounts after violations if the sellers provide corrective action plans, though the accounts would be blocked after further infractions.

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It is often hard to tell that an Amazon seller is based in China, as is the case with the Amazon page of Lagunamoon, the essential-oil and cosmetics provider. It shows no indication the products are Chinese and gives no store address. Lagunamoon’s Mr. Zhao said that is because the U.S. doesn’t require it.

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Amazon seller Molson Hart in Texas is suing 73 sellers, many located in China, in Texas federal court, for trademark infringement on products like his Brain Flakes interlocking plastic disk set. He has been selling the Chinese-made toys on Amazon since 2014, and counterfeits started appearing in 2015, he said.

After he filed suit, he couldn’t hunt down the Chinese companies. “I know who did it,” he said, “but I can’t serve them.”

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Amazon buyer Irvin R. Love Jr. of Georgia bought a hoverboard on Amazon in November 2015 that caught fire and burned down his home, according to a suit he filed February 2018 against Amazon, the seller and others, in Georgia federal court. In an amended complaint this year he alleged that Amazon was negligent for not removing the hoverboard from its website before Mr. Love’s purchase. Amazon argued in a legal filing that it doesn’t owe damages because it didn’t design, manufacture or sell the hoverboard.

Mr. Love also sued the seller, Panda Town, which his lawyer, Darren Penn, said appeared to be a Chinese company, based on sales information. Mr. Penn said that he can’t locate the seller and that Amazon declined to provide its location.

Cross-border e-commerce has made it harder to police unsafe products entering the U.S., he said. “When you had the traditional importer and customs and brokers—and all those procedures are followed—you provide a couple of layers of protection that you don’t when you’re talking about an internet market.” The case is in discovery, and Mr. Penn declined to make Mr. Love available for comment.

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“It’s not normal that a factory with 200 people manufacturing baby monitors in Dongguan can ship products directly to consumers in Minnesota or in Europe through a marketplace,” he said. “The day the regulator makes them responsible, then we’ll have proper compliance programs.”

Amazon said sellers create their own product listings and are required to comply with all relevant laws and regulations when listing items for sale in Amazon stores.

Mr. Thompson, the electronics seller, said Chinese factories have steadily pushed him out of lower-end goods such as USB cables, pricing at less than he can. The Chinese sellers often boost their product rankings by arranging large purchases of their own products and leaving positive reviews for themselves, he said—a tactic he said he learned about while attending an independent Amazon-seller event featuring a China-based sales consultant in Hong Kong several years ago.