Traded in Marvel vs. Capcom 2 a long time ago for some game that I can't even recall....not only is it one of the best fighters of all time, you now have to pay $80-100 just to get a copy. I don't even know how much I got for the game; I'd bet it was about $5
I flat out returned Tekken 1 when I saw each character only had like 2-3 moves in the instruction manual...and doing them didn't work half the time. Little did I know that the manual sucked and the timing to doing moves was very different from Street Fighter.
every time I trade a game I regret it later... Thats why I never do trade ins... I always want the games I buy... even if they suck, I still play them and build that nostalgia. A few years later I always want to pop that game in... and I don't have it available.
007 Goldeneye for Wipeout 64...worst part is I don't remember agreeing to it. I found my 007 game at a friends house and he said we traded. Which explained where the wipeout game I had never seen before came from...I think I got hosed...
Not so much a trade, but I regret not buying Ikaruga for the GC when I had the chance. They had it at a local used game store for $35 a few years back. Being a poor college student, I passed it up because I wasn't about to pay $35 for a used GC game, even that long ago. Still, I wish I had now. It's super rare, and goes for something like $70+ on eBay.
True story: I traded in my NES, SNES, N64, and a ton of games and controllers for all of them for a brand new Xbox when they came out. I paid $0.63 after tax at Funco Land. It was promptly stolen from my room at college when my idiot roommate failed to lock the door. I traded in some real gems:
Legend of Zelda (NES gold cart)
Secret of Mana
Among a bunch of others. More recently, I regretted trading Panzer Dragoon Orta, Marvel v. Capcom 2, Phantom Dust. Strangely, I haven't regretted trading in any of my current gen games.
Not really traded, but about 10 years ago I sold Dragon Force along with my Saturn and a couple other games for $100 to help fund the purchase of a Dreamcast. Loved the DC, but have missed Dragon Force ever since.