You don't own the charactors you play on, they are owned by Blizzard, the same for the account you use it is all owned by Blizzard. The only thing you are actully selling is the time you have spent on the charactors. Selling the account is against the rules of the game and you have agreed to them, however if you get the account banned then there is nothing the other player can do, he will have to admit that he is buying accounts and therefore the account is banned.