Hmmmm, have you gone into Game or Gamestation recently? The prices they charge for a lot of their used games are ridiculous and only a couple of £'s less than brand new!
And besides theres nothing worse than when they bring a used game out and then having to refuse it because the disc has got scratches all over it!!
Might as well buy games brand new and avoid the hassle, and if you choose your games carefully you wont need to trade them in anyway!!