Citizens Advice Bureau has warned of scams on eBay, Gumtree and other online market places.
It issued a warning after its consumer service bureau found one in six consumer problems it investigated from Gumtree was a scam or potential scam.
It found that one in 10 problems reported from eBay in the six months to March also involved a scam.
"These sites are an important service for buyers and sellers, but con artists are profiting from them too," said Citizens Advice chief executive Gillian Guy. "Scammers are swindling people out of out of hundreds or thousands of pounds by posting false products and services online."
In 2013 the most common fraud reported was online shopping and auction scams. This cost consumers in the UK £63.6m, according to figures from the National Fraud Intelligence Bureaux.
Citizens Advice analysed 649 Gumtree cases and 3,711 eBay cases that were reported to them between October 2013 and March 2014. Of these, it spotted a scam or potential scam in 108 Gumtree instances and 386 eBay cases.
Ms Guy said: "It’s time for online marketplaces to up their game and do more to protect their customers from dodgy dealings by strongly policing their websites, carrying out spot checks and immediately removing any risky ads.”
An eBay spokesman questioned the research, pointing out that Citizens Advice found 386 cases of alleged fraud over six months but the site receives 19m visitors each month in the UK.
"Customers who purchase on eBay are covered by the eBay Money Back Guarantee and get the item they ordered, or their money back."
Gumtree also said it takes fraud very seriously and does everything in its power to keep scammers off its website.
