umm... yeah you'll need the cd key for hl2 to play single player if you plan to run it via steam. and the key better be useable... that's mainly what you're paying for.
If its unused your fine, If its used, It won't work....You can only use 1 CD KEY PER Steam Account, so that key is already issued to X steam account, and duplicate Steam accounts with the same key don't work. Believe me, I found out the hard way..
If you must go through ebay and a used HL2, the Steam account must be sold, then PW changed.
Don't do it. Not only is it really easy to get screwed, most of those are were bought with stolen credit cards... which later get cancelled.. which means you spend money and get nothing.
Just buy the game retail or online through steam.. it will be a much smaller headache for you, I promise.