WoW account retrieval – Happens when the seller of the account retrieve it (with Secret Question most usually) when he/she received a payment.
Resolution: Blizzard should be contacted to resolve the issue.
As a conclusion, you cannot be too cautious and you must accept the fact that anyone can be a victim of the scam. That’s a sad truth. However, it’s important that we take necessary measures, because then it would be a lot easier to avoid possible frauds and scams.