I'm an obsessive collector. Why would I want to trade something in? I never know when I'll get the urge to want to play something. Besides, finding more places to put games is part of the fun! (Thinpacks are actually helping with that right now...)

I have some that are keepers.  I won't trade them in because I enjoy then so much (Oblivion, Tron Evolution, Arkham Asylum, both Endless Oceans, Dragon Age Ultimate Edition...).  But, I have made some mistakes in the past, trading  a few games that I should have kept (Enslaved and Alan Wake).  But others, I have no problems selling while their value is still high enough (Dead Space 2, Blood Stone, Shattered Dimensions).  I have no regrets for these.

And I have fun finding games for the PS2.  With those, I take my time because I know they will be keepers.  So far, I have Final Fantasy X & XII, Ico, Shadow of the Colossus, Silent Hill 3 and Beyond Good and Evil.  They are all in near-mint condition and building this collection is really cool as I feel each game is worth it.

I always give away and/or lose my games

Out of 9-10 games I had for my N64, I can only find Pokemon Stadium.

Out of 10-17 games I had for my PS1, I can only find Digimon World.

Out of 20-22 games I had for my PS2, I can only find WWE Here Comes The Pain & Resident Evil 4.

Out of 10-13 games I had for my GC, I can only find Resident Evil 0 & LOZ: Collector's Edition.

I keep all my games -- have over 1600 (physical and digital) they're kind of like my kids :P

I don't trade or sell anything unless I end up with 2 of the same game. I also keep all boxes and manuals that come with any game or console.

I keep them all! For many of the same reasons people have already expressed on this thread.

Trading in games is a rip off. I keep them all now to protest the digital download age. Same with my book library.

Nice collection! I thought I was bad. I have about 35 360 games and 7 PS3 games. Nice.

I want to keep almost all of my games. You never know when you might want to play one again. Or, one of your friends. I was thinking about getting rid of Little Big Planet, but then I got a new girlfriend that sucks at games, although she is very interested in learning to play. This game was a great way to do that with her(she likes the Kinect too, but it doesn't have that many games for it, and obviously no controller). I rarely get rid of my games. I have Dragon Age with the manual, but I didn't like it that much. I don't want to trade it in, because I will never get close to the amount of money I put into it(they never give you any money for the strategy guides too, which is crap).

I used to return or sell my games. It was a giant mistake. Haven't done so since early 2007.