I dont know if this is the right place for this thread.
This is not for everyone but many places have cex and gamestation in their town.
My brother was in town and we have a cex and gamestation on the same road.
If you buy Mortal Kombat vs. DC Universe for PS3 for £20 from gamestation, CEX will give you £27 trade-in value. (xbox 360 trade in value at £25)
He purchased 3 copies and traded them all making about £21 quid. Which in the end means he can go back and get another copy free.
This is just one example - there are other games in the range where you can profit on too but not as much as this title
Either you can just sell the one and make £7 or sell 3 and get yourself a free game this xmas :)