I trade games in because a lot these days really don't have any replay value, or don't stand out so much that they make me go "WOW! This is something that deserves to be on my personal shelf", I've actually only ever had a few games that I've traded in that I actually regretted selling and those regrets were:
Batman Arkham Asylum (I went and thought of buying another copy, but having deleted my save for extra room I decided against it, it was more of a "Deserves the shelf" thing anyway), Velvet Assassin (At the time I had a lot going on so I barely got to touch it, ended up selling it and I've been desperately searching for a new copy, but I can't find any at all! It's maddening!), Saints Row 2 (However the next week I was able to snag a new copy on sale for only $7 more than I got on the trade in, so essentially it was a trade up, also I'm Aussie, so that's around $3 in the American Games market).
However, the games that have earned themselves a permanent slot in my collection are (Excluding the ones above which would have themselves a place here):
Bioshock, Red Dead Redemption, Dragon Age (As well as it's DLC, as fucking shit as a couple were), Left 4 Dead, CoD: WaW (purely because of the fact me and my mates enjoy playing Nazi Zombies together, fantastic side-game that there is, however the rest of WaW is easily one of the most broken, dreadful, glitchy and pathetic pieces of shit I've ever dealt with to date, though I did choose to complete it on veteran), Splinter Cell: Conviction (I really do enjoy the play in this game a great deal, the conspiracy was bullshit, especially with regards to his daughters corpse, but it was fantastic fun), Dead Rising (Broken controls and godawful story, but going fucking insane in a zombie-filled mall is great and it is some balls to the walls fun, the perfect game for a 10 minute mind-fuck) Alan Wake (Many Reasons, but at the moment I'll just say it was sensational in every way, not saying it's the best, but if you can try the game, do it!) HALO 3/FABLE 2 (Purely because it was a package deal and I can't trade the fuckers in, lord knows I've tried),
I'm not listing a few games which can't be sold, like Portal, Plants vs. Zombies and the like, but even if I could, they've earned their place.