The answers simple - if u buy a 2nd hand steam game then the code has already been used !
when u try & use the code steam will deny u your game - what can u do about this ? unless u have the original receipt (ironically from within last 3 months or no good, so what about the older games ?) then nothing - steam gave me the run around over several weeks and still denied me access to my game simply because i didn't have a recent receipt for a 2 year old game even though i had the original game & serial number and sent them the proof.
i was simply told if i wanted to play the game so badly then to buy it again using online download - apparently the only valid way to play older steam games these days.
any1 know of a work around ?