Here's a believable scam I came across recently. Has it ever happened to you, perhaps without you even realising?
I won a bid at a very favourable price and was delighted with myself for a while.
After a few emails, the seller (cecilcecilcars, but could have been anyone) told me that unfortunately he/she could not supply the item because it had been damaged by the machinery in the Post Office and returned to him/her. My money was returned after a while.
Being a trusting person I believed this at first. Then I found out that the Post Office would not have done this. Post Office policy is to repackage the damaged item and include an explanatory note stating what has happened. Then it continues on its way to the addressee. Only in the most unusual of circumstances would it be returned to the sender. As cecilcecilcars had tried this excuse a few times before, I knew it was a scam.
Presumably, the plan was to relist the item later and only supply it if the winning bid was high enough.