Didn't used to sell games but now I do because they become almost worthless after a year or so and can always be bought back for a couple of pounds if I want them again.
I usually used CEX in the UK, they give a much better trade in price.
For example I took my friends daughter to trade some DS games in Yesterday and was offered:
GAME: £46 cash £62 trade in
Gamestation: ££58 cash £70 trade in
CEX: £78 cash £118 trade in