Quote Originally Posted by Nano2k View Post
Because once you register it to your account the code is not valid anymore, it becomes useless so you can't use it anymore.
Not really true, if the guy who sold you the key has the retail version or a coupon with the key printed on it he can submit a photo of the printed key as proof of ownership and valve will delete the game from your account and give it to him.
You've been scammed basically.