Online auction bidders have been given a "wake-up call" after a man was prosecuted for bidding against himself to raise prices.
Trading standards officers warn it is illegal to push up a price artificially by bidding against yourself or by getting family or friends to do so.
The process is known as shill bidding and breaks new European Union fair trading rules.
Offences carry a maximum fine of £5,000 in UK law.
Minibus hire firm boss Paul Barrett, 39, from Stanley, County Durham, pleaded guilty to 10 offences at Skipton Magistrates' Court after using two separate eBay accounts to bid on and increase the prices of his own items.
He told the court he was unaware that bidding on his own items, including a pie- and pasty-warmer, was against the law. The case was adjourned for sentencing at Bradford Crown Court on 21 May.
Claire McKinley, trading standards officer, told the court: "Mr Barrett placed bids on items he himself owned to raise the price and left positive feedback on his own eBay site, leading buyers to believe his reputation was better than was the case."
Jo Boutflower, of North Yorkshire Trading Standards, said: "I think people do it either themselves or by getting friends and family to bid on their items and do not think they are doing anything wrong, but actually they are breaking the law.
"We certainly hope this case is a bit of a wake-up call to people who do trade on eBay, or other auction sites."
Speaking after the hearing on 16 April, Vanessa Canzini of eBay said: "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."