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Trade Me members have been ripped off by foreign scammers hacking into the online auction and selling non-existent cars.
Spokesman Dean Winter said five customers had lost a total of $40,000 after sending money overseas via Western Union.
The fraudsters, who are believed to be based in Britain, pulled off the sting by phishing - taking over accounts, changing passwords and listing fake items - usually cars at half their value.
"They will say they are offshore but the vehicle is in New Zealand but you won't be able to go and view it."
Winter said users should be wary if no registration number was listed and only pay money only into New Zealand bank accounts.
"Do not be convinced to deal with anybody trying to get you to put money into an overseas account.
"And if the deal sounds too good to be true, it probably is." Winter said Trade Me, which has two million Kiwi members, was making continued efforts to trace the scammers.
Dozens of customers, including anyone who had viewed one of the offending pages, had received warnings from support staff.
Winter said 99 per cent of potential scammers never made it past the site's comprehensive security measures, which included a 24-hour monitoring team to follow up breaches.
He said the number of fake listings was tiny, with more than 178,000 cars sold successfully through Trade Me in the past year, but one such incident was "one too many".
