The Consumer Protection from Unfair Trading Regulation 2008 act makes it a criminal offence for traders to make any misleading claims.
This includes claiming to be a consumer making private sale or claiming to sell branded goods that are not the genuine article.
The regulation, which implements the 2005 European unfair commercial practices directive, could affect online sites themselves as well as sellers, according to Dai Davis, partner at legal firm Brooke North.
"The regulation will prove to be a real challenge to online trading sites like e-bay if it is properly enforced because it will be difficult for such sites to prove they have taken all reasonable precautions and exercised all due diligence to ensure claims made by sellers are not misleading," he said.
The changes to the old Trade Descriptions Act also make it illegal for traders to advertise goods for sale unless the trader is able to meet market demand.
Davis said manufacturers and retailers will need to be more careful about the timing of marketing campaigns to ensure they have stock available before advertising.
End-users who suspect online trading information is misleading should report it to the trading standards office responsible for enforcing the trade law, he said.
E-bay did not wish to comment on the issue.