Britons selling goods on internet sites such as eBay are being targeted by scammers who try to dupe people into sending their items abroad before they ever receive payment.

Security experts said that African countries are one of the main sources of the scam, as fraudsters move on from traditional e-mail schemes designed to trick people into handing over cash or credit card details. As internet users have become aware of the e-mail racket, it is believed that criminals are turning their attention to tracking people on the web who are selling high-value products such as laptops, mobile phones and digital cameras.

Evidence seen by The Times suggests a pattern to the new “phishing” scam has emerged. Items offered for sale are bought at a higher price than usual. The seller is then emailed by the buyer to say that cash has been transferred. Then another message from a supposedly reliable source, such as Paypal, a trusted online payment service, suggests the payment is pending - but that e-mail is actually a fake.

The delivery address for the goods is usually in an African country such as Nigeria. If sellers do not post an item immediately, they begin to receive increasingly abusive and angry e-mails in an attempt to bully them into mailing their products.

“They look for you to courier your object over,” said Rik Ferguson an analyst from Trend Micro, the internet security specialists. “Once you do that, you’ve been scammed. They drop their e-mail address, you don’t hear back from them and you never get your item back.”

Sources suggest hundreds of people have been affected by the con on eBay. But the problem is thought to be more widespread, also affecting other sites such as Gumtree, where sellers have reported similar issues. Consumer electronics are often targeted because they are easy to sell on.

EBay said it has been aware of such schemes since 2006, though security experts claimed they have grown rapidly in the past year.

John Jones, 44, a worker on a government website from Welwyn Garden City, said he had received a generous offer for a new mobile phone he was selling on eBay. He became suspicious when the buyer, apparently a woman living in America, asked for the package to be sent to her boyfriend in Nigeria.

“I received a Paypal e-mail saying suggesting I should send it immediately. I thought it must be a scam, so I didn’t.

“Then the e-mails started to become abusive. They were a bit startling. I thought, 'Blimey, this is moving to bullying and intimidation.' I know that some people who receive an e-mail saying there is a legal obligation to send something might panic and send it.”

One 35-year-old woman from London, who asked not to be named, said she had a similar experience with a man from Nigeria trying to buy a laptop she was attempting to sell on Gumtree. She insisted that she could not send anything without receiving proof of payment first. In response, she received e-mails saying, “Don’t get me angry with you”, and threatening to contact the FBI unless she sent the device.

This week, Nigeria’s anti-corruption police said it had joined forces with leading technology companies such as Microsoft to investigate and shut down e-mail fraud gangs based in the country. Millions of internet users have received messages that originate there, offering huge financial rewards in return for assistance in releasing money trapped in a foreign bank account.

Though the majority of these e-mails are instantly deleted, the fraudsters still find a sufficient number of gullible Britons to make it worth their while, to the tune of millions of pounds per year.

An eBay spokeswoman said that they were aware of the new trick, and always advised customers to double-check their personal accounts to see if payments have been made before sending products.

Mr Ferguson, from Trend Micro, said: “If a stranger knocks on your door, and says, ‘I hear that you’re selling your car, give me the keys and someone else will pay you tomorrow’, you would never have that. The same logic applies online.”

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