I trade in games but only if i know for a fact ill never ever miss playing them again. Games that I love and I know i will come back to I keep, even if its 2 years before i go back to them. Ive got a gamecube solely so i can play metal gear solid: the twin snakes. While some of the dialogue and music has been changed (some better, some worse) the upgraded graphics and gameplay make it worthwhile when im doing a runtrhough of the entire series. Ive still kept the original game but when i play the MGS series i always play Twin Snakes now (any scene with the ninja is now 100 times better).
The only game ive traded in that i regret (but thankfully when the time comes i can buy it back brand new for less than a tenner) is Deadspace. i played it once, and never thought id go back to it because im not much of a survival horror fan. Then several times since ive had the urge to play it again, and with the sequel coming im sure i will.
So of the games i currently own i have
PSOne:
Metal Gear Solid
Fear Effect
Driver
GTA
Dino Crisis 1 & 2
Abes Oddysee / Abes Exoddus
PS2:
Metal Gear Solid 2 & 3
GTA: Vice City
Ico
Shadow of The Colossus
PS3:
Metal Gear Solid 4
GTA IV
Ghostbusters
Bioshock 1 & 2
Batman: AA
Uncharted 1 & 2
AVP
Red Dead Redemption (yet to play it. got sidetracked by MGS: Peacewalker)
Chronicles of Riddick
Heavenly Sword
God Of War 1,2 & 3
The Force Unleashed
PSP:
God Of War: Chains of Olympus
MGS: Peace Walker