A seller on eBay who drove up prices for goods by secretly bidding for them faces fines of up to £50,000.
Paul Barrett used a second account on the internet auction site to also post positive feedback.
The company said the case, the first prosecution of its kind in the UK, was a clear warning against the practice of "shill bidding" to bump up prices.
Barrett, 39, who runs a minibus hire firm in Stanley, County Durham, admitted 10 charges of false bidding for items ranging from a Range Rover to a pie and pasty warmer. He was caught after a separate complaint over false mileage on a minibus prompted an investigation into his dealings by North Yorkshire trading standards officers.
Skipton magistrates heard that Barrett had set up an account with the user name Shanconpaul. But he placed bids using a different identity, Paulthebusman. Trading standards officer Claire McKinley said the ploy increased the price of dozens of items, including mobile phones, a cash register and a Mercedes car.
"He raised the price and left positive feedback on his own eBay site, leading buyers to believe his reputation was better than was the case," McKinley said.
Barrett, who was prosecuted under the Business Protection from Misleading Marketing Regulations and Consumer Protection from Unfair Trading Regulations 2008, said he did not realise he was breaking the law.
"eBay let me open the second account and I gave all my personal details and home address to do so," he told the magistrate.
"I realised the price was too low on some things and put the prices up using the second account. I've never been in trouble before and would like to apologise." The two regulations, which became law after growing complaints about internet fraud, carry fines of up to £5,000 for each offence.
Jo Boutflower, head of enforcement, intelligence and process at North Yorkshire trading standards, said: "We hope this case is a wake-up call to people who trade on eBay, or other auction sites. Some people do this either themselves or by getting friends and family to bid on their items. They may not think they're doing anything wrong but actually they are breaking the law."
Vanessa Canzini, head of corporate communications for eBay in Europe, said: "We are extremely pleased to learn of the conviction of Paul Barrett. Shill bidding is illegal and it is important for people to understand that there is not, nor has there ever been, room for illegal activity on our site.
"We invest over £6m each year in state-of-the-art technology to detect shill bidding and other illegal activities. This acts as a strong deterrent to the small minority who attempt to use our site inappropriately but, more than this, it helps us to work with law enforcement agencies to secure successful prosecutions if anyone decides to try their luck."
The case was adjourned for sentencing at Bradford crown court on 21 May.