If an internet auction goes sour, think twice before posting abuse about the seller - legal experts say you could be sued for defamation.

The warning follows a $21,000 lawsuit in the United States after a disgruntled buyer left negative feedback on a seller's eBay account.

Kiwi lawyers said the principles of defamation are similar in New Zealand and the US and such action could happen here.

"The law is no different if someone writes it it online or in a newspaper," said William Akel of law firm Simpson Grierson.

"Text messages, emails can all be used to defame someone."

About 215,000 pieces of feedback are left on Trade Me each week. About 15 are referred to staff and two complaints upheld after investigations.

A spokesman said the company wasn't aware of any complaints making it to the courts but kept a "trail of who said what to who" - just in case.

A survey of Trade Me feedback revealed a string of comments lawyers said could be defamatory, from the relatively benign "trader is not honest", to the more extreme, such as "What a freakin fruit loop".

While it might be possible to take a lawsuit, David Campbell from Campbell Law said such a move was unlikely to become commonplace.

"People get away with an awful lot online and it's been going on for a while but I haven't seen a flood of complaints. We don't live in a litigious environment."

Victoria University defamation expert Professor Bill Atkin said the online community should remember "the wisest course is often the cautious one".

"I think many of the things people say on Trade Me, blogs, Twitter, Facebook, could be defamatory."

Trade Me has a policing team working around the clock to stop a range of offences, including offensive feedback.

It also has a Community Watch service, allowing members to fast-track concerns and which carries information about its feedback policy.

The company also lets people know that they can be held legally responsible for damage to a trader's reputation, and that it's important to ensure feedback is factual.

What is defamation?

A defamatory statement is one that tends to lower the person in the estimation of right-thinking members of society, or which tends to cause him or her to be shunned or avoided, or which tends to cause a person to be exposed to hatred, contempt or ridicule, or which is a false statement about a person to his or her discredit.

Source: Bell Gully