As far as my collection goes, I would never sell something from a set I'm collecting on. For instance, I just sold Okami for PS2 because I didn't like it. It will probably be worth quite a bit sometime down the road because it was a black label, but I'm not collecting PS2 so I sold it. It was foolish really, because eventually I probably WILL be collecting PS2 and I will regret it.
But I have a lot of sports games for SNES and I wouldn't sell those (as if they're even worth anything

) because even though I'm not actively trying to get every single game, once I get some of the pricier stuff out of the way and have every game I want, I might try for a full collection. I even went out of my way to buy a sports game for the Genesis the other day JUST because it was CIB and only .50 cents for that same reason.
As far as the Xbox 360, PS3 and Wii stuff I sell it off when I'm done with it or when I don't like it. That's dumb too though, because 50% of the time I buy games I sold back to replay them. I have a hard time learning from my mistakes.
