this gen...

ninja gaiden 2 (too repetative)
stranglehold (i enjoyed the demo, thats why i bought it..but then it was just the same thing over and over again)
ratchet and clank: ToD (not nearly as good as the first 2 games IMO)
super princess peach (very shallow gameplay)
GTA IV (havn't liked a GTA since GTA III)
The Bigs (i liked the demo, but i like The Show 100X more)
God of War: CoO (i love the PS2 GoW's but CoO was only 5+ hours and not as grand as the other 2)
Patapon (i don't think i understood how to play the game lol)
PSP (i know it's not a game, but i bought it twice and traded it in twice..game library is horrible IMO)

Resistance 2, Shenmue 2 (xbox), Morrowind, Command and Conquer 3, No More Heroes, Forza Motorsport, Boom Blox.

I only buy games if I'm 100% certain that they're good. All of the games listed above were supposed be good, but turned out to be trash.

Haze

Only once in my entire life

Star Wars: Force Unleashed (Wii).

I traded it in for a new copy of MASS EFFECT.

quite possibly the best decision I ever made in my life.

This.

EDIT: I never trade in/sell games though.

No More Heroes, Pangya went back to the shop pretty quickly.

Worst purchase was Strongbad Ep 1 on WW. I hate it but I can't take it back!!!

Metal Gear Solid 4. It was like a really bad movie.

Only game I've ever sold was a game called "NFL" for the NES. It was a crappy copy of tecmo bowl and i sold it for $5 10 years ago. I later bought it back for $2 at gamestop (back when they used to have NES stuff) and it was just as bad as i remember it.

I don't get too upset with most games, but Black & White was so bad I had to get rid of it asap.

Eternity's Child (YES I BOUGHT IT), but I can't get rid of it...