I had a buyer recently who, two weeks after paying by Paypal and receiving the goods, and giving positive (though slightly odd) feedback, requested a Paypal refund because she had also apparently paid me by Western Union.
It turned out that somebody had contacted her after the auction and, very convincingly, explained that for some reason the Paypal payment was rejected, and asked her to pay by Western Union direct money transfer (to a certain Carla Mitchell based in Madrid - who fares badly in terms of reputation in Google's search results).
She did so, the fraudster collected the cash, and she was left seriously out of pocket, having paid £200 twice.

You might think that sending money to a woman in Spain by an untraceable payment method, when the seller (me) is a man in England, is obviously a scam but this one was very well done, and it's a reminder to us all not to let our guard down when dealing with online payments.

No genuine seller will ever ask for a money transfer payment via Western Union or similar - it is not permitted by ebay rules, and it is not a secure or traceable payment method.