If you have the account's information, then call up Blizzard and claim that it got hacked and have the email and other information changed again. However, if the account's name is under the person that "hacked" your account, then they will have the final say towards ownership, since they can just fax in forms and claim the account. You can file a dispute or unauthorized claim on paypal to screw the seller, or just simply chargeback if you used a credit card.