She said that the issue had caused “certain posts with content that violated our policies” to be visible to users.

“We are working to fix the problem and will be closely monitoring our systems to ensure we are properly identifying and removing violations before giving more people access to Marketplace,” she said. “We apologize for this issue.”

In her blog post Monday morning, Ms. Ku said that Marketplace had been introduced as the company recognized that its users had been buying and selling products and services through Facebook groups. She wrote that “more than 450 million people visit buy and sell groups each month.”

Users who bought and sold products within groups had also violated the website’s commerce policy. In July, Mike Monteiro, a web designer who started a campaign to help monitor the social network for sales of guns, said that he had reported about 500 posts or groups that violated Facebook’s ban on weapons sales in the past month, and that the website had taken down only about two-thirds of them.