Seems I'm in the minority here. I don't keep ANY games and I ALWAYS trade them in at Gamestop. I really don't care how much I get back. I could afford the games I buy at full price, so knocking $15-$20 off the price is just a "bonus."
I hate selling on eBay. Too much work for me :)
My End of 2008 Hardware Predictions (console only):
Wii : 50 million
360: 28 million
PS3: 24 million
These predictions were made on January 3rd and won't be revised