So they cared to write individual serial numbers on each copy of the game eh. That's like Windows, but Windows is not just a game. I've bought a lot of games but none had serial numbers.
Really? 'cause CD-keys have been around for over 10 years. They used to be able to control who was online earlier in internet gaming history. Exemple: Diablo 1 (1997) had the cd-detection method you describe to make sure you were playing with your copy, but if somehow you managed to play without a CD (i.e. take the CD out after booting the game...), your CD-key would prevent another person to join Battle.net (only one copy of your CD-key could be on Battle.net, the internet Diablo system, at once). Of course this did nothing to prevent many copies playing singleplayer, at different places at the same time so it has evolved into what we know today. I think it even verified CD-keys for modem play... Although you could install "spawn" versions of the game to play multiplayer (i.e. a copy that couldn't play single player, it was only there to play MP with a friend using his copy).
If I know and trust the person I give or sell the game to, I could share account with him or her. Probably not a Steam approved approach, but I guess it could work.
At this point you might as well share the account with that person for free since you are so close to them. I can't think of anyone that I would try to make money off them and at the same time trust them enough with my Steam account, which has over 70 games in it. You're sharing more than your game, you're sharing your whole account. If you never intend on using your account for anything other than that one installation of Half-Life 2, well whatever I guess it's fine, but it's a really backwards way of going around the problem. You might as well create a different e-mail address and Steam account for every game you buy, although it'll be a headache. And good luck selling your game to other people who might not wanna deal with a different steam account only for your game.
Steam exclusive games are more and more common (like Civ 5!) and, well, wnat it or not, you'll probably end up with at least a few games on your steam account, which will cause all kinds of problem when someone you sold your game to tries to play his game when you are already at your own home playing Civ 5 or something like that. Without mentionning the fact that he could just play Civ 5 since, you know, he's logging into your account.
I dunno, I just consider it non resellable. I don't approve, but at this point that's what we're buying.