Because the game is linked to your account you can't sell it on, and I don't think anyone would buy it.
Even if you had good intentions to give them your log in details to the steam account linked to your copy of L4D2, who would trust you to do so?
And I'm not sure if steam would be so willing to allow you to pass on your details to somebody new, as they would not get any money from the transaction.
In terms of second hand copies of HL2, if you saw them in the store, it was probably passed on to some store clerk who doesn't have the first ideas how PC games work.
If you saw it online, it could be a scam (Which relates to my first point).
Unfortunately its the way with most PC games.