I used to trade stuff in, back in my poorer days. My brother still does it; he actually owns something like 2 games at a time, and they're always brand new.
I swore I'd never tell anyone this, but:
The thing that turned me off of trade-ins forever was this.
Many moons ago, I found a PS2 copy of Rez at Gamestop. It was already several years old, but was still priced at $29.99. I saved up for it and bought it.
I took it home, loved it, played it over and over. I considered it the jewel of my game collection, such as it was.
Four years later, I'm married. I desperately want my wife to share one of my main interests with me, and she already kind of plays games, but we never play anything together, and I really want to. (For some reason, she won't even try MMOs; she's a Tetris-Sims-Columns kind of gamer.)
I'm totally broke, so I decide I'll trade some games in and hopefully pick up something decent for PS2 that's simultaneous 2-player for like $20. I go through my collection, pick out a few choice games, and call Gamestop to check trade-in rates.
I end up trading 5 games, INCLUDING Rez to get $20. It's fucking rediculous. They gave me SIX FUCKING DOLLARS for Rez.
Now, two years later, whenever I go to that Gamestop, I see my copy of Rez on the shelf. I can't believe it's been there this long. It sits in the used section, accusing me, my public shame.
They gave me $6... but they're selling it for $30.
I'll never trade a game in again.