It's the 30th of September 2010 and I've already had $50 worth of play out of it. Even if I hadn't liked it, it has still given me value for money.
Congrats, you have been successfull contagioned by the industrie's body of thought. (Oh and - btw. just because YOU had your $50 worth of play out of it noone else should be able to sell his game if he didn't?)
I remember the days when customer had rights. Let it be a music-CD. You see it in the store and you buy it without pre-listening to it (maybe you don't have that much time to spare). You go home, you hear it. You hear it twice. It bores you, so you sell it.
What's the difference to a PC-game like Civ?
You see "Civilization", you don't think long about it, you don't read any forums, you just buy it.
You install it and (thanks to bloody Steam) create an account and register it. Then you play it, and - oh wonder - it bores you.
Why should one NOT be able to sell it, just like the CD?
My advice to the OP: If you created a Steam-Account just for Civ5, log into it, change your personal data to something useless, sell the game and the account with it.
But do NOT ask Valve if you are allowed to - because you are not. You are not breaking any laws, you just violate the EULA of Valve. *shrugs*
Oh, and try to don't get caught by doing so - they will ban the account and you (or the person you sell the game and account to) won't be able to play any longer and your $50 are lost - no matter if the few hours you played it were worth the money or not.