Retail boxed Steam activated games: can they be resold?

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.

Once you add the key, the game gets permanently tied to your Steam account, and cannot be removed. You can't give it away or sell it (or rather, you can give away or sell the box and everything that came with it, but the recipient won't be able to play the game without obtaining an unused key).

No, these games will not be able to be used by another person. Because the games required a Steam registration code and use Steam for their DRM, they are now associated to your Steam account.

In this Support article, they state:

Important:

A CD Key may only be registered to one Steam account. CD Keys are associated with your Steam Account, not your computer.