Just wanted to let you know of this, because it really makes me worried. I bought a game from a co-worker, HL2, because he had it twice (because of some orange box thingy or something). I am not allowed to play this game according to Valve, I contacted support, here is the answer from them:

Message by Support Tech Jimmy on Thu, 23rd May 2013 8:10 am

Thank you for contacting Steam Support.

We apologize for the delay.

The CD Key you have provided has been previously registered to and played by another Steam account.

Unfortunately, used CD Keys cannot be activated via Steam. This includes used CD Keys received as gifts, resold by an individual or purchased from a store selling used goods.

The game can only be played using the account on which it was registered.

If you have recently purchased this product, please contact the seller to discuss a refund or exchange.

We apologize for the inconvenience and appreciate your understanding.

If you have any further questions, please let us know - we will be happy to assist you.

I thought they were different Let this be a warning for you if you plan to buy some used games.