I'd be real leery of buying any game used that required an activation. I was really interested in picking up Bioshock but not a big fan of paying full price for it. I checked around on Ebay and saw quite a few copies for sale used at around $15-20.
I emailed maybe 5 or 6 people to ask them whether or not it had been activated online, explaining that if it's tied to their Steam account that unless they turned over their whole Steam account, Bioshock wouldn't work. Out of that 5 or 6 people I emailed, not a single one of them got back to me.
Just doesn't seem worth the risk of dropping the cash for an opened game on Ebay, then when you get hosed they can either plead ignorance or say they were only sold as backup copies for the game you currently own. It's also a relatively small savings in the long run, so they're probably just banking on people sucking it up and not complaining.
Edit: Just to clarify how Steam works ... basically, if you want to sell a used Steam game, you need to sell every single game you've ever bought that used Steam to play. That Steam account is the only thing that lets you play it, so if you've got Bioshock, the Orange Box, and Titan Quest, you only have access to them because of your Steam account. As soon as you give someone your account info, they get access to all your Steam games in your name, and when they change the password you lose that access.