The Prof William Brown gold medallion at the centre of the fraud perpetrated on J.S. Auctions.

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Yet again the fraud seemed to originate in the London area, in Potters Bar, and the firm based in Bodicote, near Banbury in Oxfordshire, were left more than £3000 out of pocket after three lots of gold were targeted.

Most were sovereigns, but Mr Smith is appealing for anyone with information about the rarest item to let the police know, although he fears it has already been melted down.

The Prof William Brown gold medallion is inscribed with the name of the winner: Stephen Keymer Johnson for a Latin ode and is in its original presentation case.

"We executed the bid, emailed the guy to let him know we'd bought it and he paid for it over the phone," said Mr Smith. "The card transaction went through OK, security to check matched, and we assumed because it had gone through there wouldn't be a problem.

"The 'buyer' sent a courier down to collect it - I was a little dubious about a courier collecting it so I then rang the buyer, just to confirm that. He said yes, no problem, I've sent my courier down. I let the courier take them away and next morning got a call from the owner of the credit card saying it wasn't him..."

The fraud reinforces the message that dealers and auctioneers should investigate exactly how their credit card terminals work and what they assess.

Mr Smith added: "It is very important that people double check that the credit card machine says data match and not just security match - that's where we were caught.

"We had always assumed if one or the other didn't match - the address numbers or the security code numbers - it wouldn't go through. But apparently it does."

Keeping alert to the risk can save you thousands, as Mr Smith points out.

"We did also have another buyer who tried to pay several times with a credit card, who was in the same locality as the other buyer, and all his credit cards were refused. As soon as I brought up the fact that we needed to confirm his address, he hung up the phone and we haven't been able to contact him since."

If you have information about the fraud contact Thames Valley police at Banbury on 08458 505505, quoting crime reference URN 673.