The adverts looked legitimate, but once customers confirmed their interest they were directed to a fake website that was supposed to look after their cash until the car was shipped, she said.
However, buyers never received a car and the money went straight into the bank accounts of the fraudsters, our correspondent added.
The Met Police confirmed that in 2008, 200 people in the UK said they had been defrauded after responding to bogus car advertisements. Officers said if they had not shut down as many as they had, those adverts would have earned the fraudsters over £150m.
Nick, from London, lost £15,000 when he tried to buy a car online from a man who claimed to be a businessman who had moved to Spain and no longer needed his vehicle.
When he asked why the bank account details were not in the name of the company named on the website, he was told in Spain it was "very expensive to set up corporate bank accounts".
"They spend their entire lives doing it, so they know exactly how to get it through as easy as possible," said Nick. "It made last year one of the worst years of my life."
Keiren Puffett, editor of consumer journal Parkers, told the BBC he felt more should be done to protect consumers like Nick.
"I would love to see far more warnings directly by the actual classified ads, warning people that there are some scam adverts about and here's how you report it," he said.
"But of course, for these classified ad operators, they just want the ads, they don't want to go into that kind of detail - it's more work and hassle."
But Adrian Black, chairman of the Vehicle Safe Trading Advisory Group, said companies were trying to combat fraud.
"I would like to reassure everyone that the leading players in used car advertising work very closely both with each other and with the police and the Office of Fair Trading to ensure we identify and stop as many of these scams as possible."
Meanwhile, a separate survey has suggested that fraudsters are continuing to target UK consumers with internet scams.
Some 77% of people asked in a poll by financial protection company CPP said they had received e-mails from fraudsters that appeared to be from a bank and attempted to extract bank account details and passwords.
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