Amazon says toy sellers have to be safety compliant if they want to sell on its marketplace. But some sellers are not asked to submit safety documents until weeks after they start selling, leaving Amazon's marketplace open to potentially unsafe products, CNBC has learned.

Amazon reached out to a group of new toy sellers in recent weeks, asking them to submit the "required safety documentation" for toys that were already available for sale, according to an email seen by CNBC. Amazon said the submissions had to be made no later than September 9th, 2019 — roughly two weeks after these sellers started selling those products. The sellers who spoke to CNBC said they were not asked to submit the safety documents prior to listing on the site. Several sellers have previously mentioned receiving the same type of email in Amazon's seller forum.

The email highlights a potential loophole in Amazon's product safety practices, which have come under the spotlight following a recent report by the Wall Street Journal that found over 4,000 unsafe or federally banned products for sale on Amazon's marketplace, including certain children's toys with high lead levels. The gap between selling and checking for safety compliance could contribute to a proliferation of unsafe products on Amazon, experts say.

"This really speaks to Amazon's 'move first and fix later' mentality," said Garrett Bluhm, founder of Vendient, a consulting agency for online sellers.

Amazon's third-party marketplace is where independent vendors sell products directly to Amazon customers. Almost 60% of all sales volume on Amazon came from third-party products last year, up from just 30% in 2008. Amazon had over 2.5 million active third-party sellers by the end of last year, adding roughly 3,400 new sellers on average every day, according to Marketplace Pulse.

The massive growth of the third-party marketplace has helped Amazon own the U.S. e-commerce market, but it's also proven hard for Amazon to police effectively, leading to problems such as counterfeits and fake reviews. The WSJ article exposed how Amazon's marketplace is not doing enough to protect consumers from potential health or safety risks, as thousands of products failed to meet safety compliance or lacked warning labels.

Amazon's spokesperson told CNBC that the company requests safety documentation "very shortly" after a product is listed by a third-party seller. The recent requests for documentation are part of Amazon's "long-standing Product Safety activities" and are "not a new program or in reaction to the Wall Street Journal article," the spokesperson said.

"All products offered in our stores must comply with applicable laws and regulations, and we regularly contact selling partners to request safety documentation to help ensure the products in our store meet the Consumer Product Safety Commission's safety standards," Amazon said in an email statement.

Sellers who fail to submit the required safety documentation will be removed immediately, the spokesperson said. Without specifying, the spokesperson also said Amazon asks for compliance documents for some product categories before listing a product. In a blog post published after the WSJ article, Amazon said that it blocked more than 3 billion suspect listings for "various forms of abuse, including non-compliance, before they were published to our store."