It has happened to me a handfull of times including quite recently. My usual modus operandi is to explain to the buyer that it is an ebay glitch and refund their money, without cancelling the transaction. It is annoying as I lose the ebay commission, but better than coping a defect for out of stock. It is not worth the agravation of phoning CS and spending hours trying to get them to sort it. .......Lifes too short for that sort of stuff. I just write it off as another bad ebay joke and get on with it.