I haven't bought a used game yet. Generally, used prices are only a few quid cheaper close to the game's release, mint condition cannot be guaranteed and buying games used doesn't support the developer. Even months after release, some games can be cheaper new than pre-owned, if you find the right online store with the right deal.
I also never trade my games in, which is why my loft is full of old games from when I was a kid.
Trading games at the likes of Gamestop is a complete joke. They give you 10 to 20 percent the amount they'll resell it for, if you're lucky. I remember just a couple weeks ago I saw someone trying to trade in their PS3 at the local gamestop, and they could only get something like $45 cash or $60 store credit. Meanwhile, they're selling the PS3 used for what, $120-$150 or something like that? Wtf?
The only benefit of trading IMHO is if you have TON of stuff to get rid off, want a little fast cash, and are too lazy to deal with selling on Ebay or Amazon or Craigslist. Otherwise it's a complete ripoff.