Prospective tenants are being conned out of thousands of pounds in an elaborate internet scam, a BBC London investigation has found.
Conmen put exclusive properties at low rents on advertising website Gumtree then trick payments from would-be tenants via money transfer agencies.
BBC London found the e-mail addresses of the scammers originated in Nigeria.
A spokesman for Gumtree said it was working with the money transfer agency Western Union to prevent the fraud.
Sisters Alexandra, 19, and Zaire Sheppard, 18, from Holloway in north London, have been victims of the scam.
They spotted an advert on Gumtree for a two-bedroom apartment in Belgrove Street in Camden. The cost was £650 a month, plus a deposit of £700.
I was put off my guard because I was not asked to send the money directly to Lin Dong - I sent the money to my sister
They e-mailed the owner, who called himself "Lin Dong".
He claimed he had had trouble with tenants before and wanted proof the sisters could afford the rent and deposit.
Lin Dong asked Alexandra to send a payment of £1,350 through Western Union, not to him, but to a trusted friend or relative of her choice.
He then requested a scanned copy of the transfer payment receipt so that he could verify that she had the available funds.
Money transfer agencies are being used to complete the fraud
Some time afterwards, a fraudster using fake identification, pretending to be Zaire Sheppard, walked into a Western Union office with the scanned receipt and took the money.
Alexendra said: "I was put off my guard because I was not asked to send the money directly to Lin Dong - I sent the money to my sister."
BBC London found a similar scam in which a fraudster was offering a two-bedroom flat for rent in Wigmore Street, Marylebone Village.
The fake owners, who identified themselves as "Katie" and "Benson", claimed they were on holiday in France and needed a quick deal.
As in the scam which hit the Sheppard sisters, they wanted £1,350 to be forwarded through Western Union and then to be sent a scan of the receipt.
But BBC London checked the origin of the e-mails and found they were all sent from Lagos, Nigeria.
During a telephone conversation, a man who called himself "Ben" said the flat was still available.
He was asked for an exact address in Wigmore Street but refused to give it. He then said he was in Paris.
When asked if he meant Lagos he hung up.
A Gumtree spokesman said: "We strongly discourage users from transferring money in advance of seeing a flat, or anything else they may see advertised on the site.
"We also operate a community reporting system whereby users can easily alert us to suspicious activity so that we can take action, and furthermore we are working with Western Union to try to put a stop to this sort of fraudulent activity."
Western Union said it was investigating the scam, but could not offer refunds to the people who have been defrauded by it.
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