I recently purchased a second hand copy of Call of Duty Black Ops. I have tried to install it via steam but when it asks me to input the game key it tells me that the game key has already been used under a different account to mine. Is there any way round this??
Welcome to the Arqade SE!
Unfortunately, when it comes to disc based PC games requiring a Product Key / CD Key, there is no legal way around this. A product key may only be used once as it is used to essentially "register" the game as yours, or in this case, the other persons.
Sorry about that! :(