So this is a pc game then. Then you shouldn't be able to sell it since the disk is link to your account. I not sure but some programs and games have a deactivate function. You should ask game stop or the actual game company and see if you can. Due to having an account linked to the actual disc it makes it virtually impossible to sell the actual disc when you do finally get bored of the game. A shady practice by the MMO gaming industry, but one that is quite often overlooked.
So why is this even happening? After all, developers and publishers allow you to sell console games. The music industry allows you to sell your long forgotten CDs. Even the movie industry allows you to sell movies to your friends. Why not your pc games.