Amazon is the world's largest online retailer. The site sells everything from mattresses to over-the-counter drugs. Recently, a Wall Street Journal report raised concerns that some products sold by the company are knock-offs.

So how can counterfeits slip into the system?

There are multiple ways that retailers sell products on Amazon, CBS News correspondent Vinita Nair reported. They can list items on Amazon's website and send them to customers directly or they can send products to Amazon warehouses and Amazon will send them out. But brand name companies say both of these systems are allowing counterfeiters to sell knockoffs to unsuspecting customers.

Matthew Frank is chief executive officer of ICI USA. His company makes Tovolo kitchenware, which are sold by various retailers nationwide. But in recent years, he says some customers that have bought his items through Amazon did not receive his Tovolo products.

Frank said when customers receive knock-offs, it hurts his brand.

"We have had consumers that have written negative reviews on Amazon where we've been able to get that product back from the consumer, and we know it's not our product," Frank said.

Jeff Messer, general manager of Retail Solutions, Channel IQ, said, "We've talked to dozens of our manufacturing clients who are really dealing with three key issues. It's counterfeit, used being sold as new, and what we'll generally call defective goods."

Messer consults for hundreds of online retailers. He says this is about more than just one company -- and more than just kitchen wares.

"We have medicines, for example, that are expired or have been tampered with," Messer said.

He says part of the problem is that Amazon allows many different companies to sell a brand's products, and if a counterfeiter lists an item on Amazon's website, the safety checks in place aren't always strong enough to stop them from selling knock-offs to a customer.

But Messer says even customers that buy directly from Amazon are at risk of receiving fake goods. That's because counterfeiters are sending knock-offs to Amazon's warehouses where they can be mixed in with real products.

Amazon would not give us an interview for this story, but provided a statement. It reads, "We are constantly innovating...to improve the ways we detect and prevent counterfeit products from reaching our marketplace. We work hard on this issue every day..."

Amazon's website says by mixing different shipments of the same products in its warehouse, it can get items to customers faster and reduce labeling requirements for sellers. Amazon does offer refunds for products that aren't as advertised.