No you cannot. This is part of the agreement you agreed on when you linked the game to your account. Therefor is also cannot be regarded unfair. As the comment suggests though there are a few new interesting developments, but so far no one really knows what they mean. Hopefully we will be able to force steam (and all other companies) to enable us to sell what we own, but for now, we cannot.

Evidence of this provided by Joseph Kendall: Steam Support Link

As the other answers have stated, once the game is registered with an account, it's tied to it and that's that.

However, another option you have that I've read about is to create a new email address and steam account for every game you get and register that game with that address. When you are ready to get rid of it, you can sell the steam and email account.

All in all, not very convenient, but it works.