This happens - I had an sale recently where there had been several bids including several from the winning bidder. Within minutes of the auction ending I had a message from the buyer saying simply "don't want it..." nothing more, no sorry I bid in error, etc, etc.

I offered a 2nd chance offer to the next highest bidder which fortunately was accepted but these time wasters can just mess sellers around and there's nothing that can be done, eBay won't block them nor can you leave negative feedback.