GAME also sell preowned copies for £7.99
This does work as i have just got back from argos.
Purchase a copy of my sims on the wii for £10 from cex, and the head over to argos where you can trade it for a £25 instore voucher. Luckily the CS aren't too clued up on games and as such are just working off their price list.
Worked in both my local stores as i managed to get 2 copies from my local cex, Nice quick £30 profit for me, albeit it in argos vouchers