Hi,
Razor's right: unfortunately the way CEX is operating nationally, it tends to now offer trades/exchanges only, and cash is almost unheard of.
The amount of times I have been told in the past (by their Customer Service phone peeps), that they are doing cash, and then when you turn-up to the store, they say they aren't, is farcical! What annoys me more though, is the fact that they keep displaying posters and mini advertising boards outside of the stores themselves, saying that you can sell CEX your unwanted computer software and DVD's
for cash!
I've even had a couple of occasions when I've reluctantly come back in the following day, having been assured by CEX staff that they absolutely will have cash waiting for me for my goods, only to turn up and find that they haven't got any cash, because nobody has bought anything yet!
Unfortunately, other than Blockbuster (who offer you peanuts for DVD's) and secondhand shops, CEX are the only company who offer a reasonable price for secondhand discs/sets.

Blockies will generally offer you £2 on your account, or £1 cash, no matter what the disc, or about 75p per disc, for all boxed sets, again, irrespective of the original cost or quality of the set being sold (e.g. James Bond Briefcase set, with 20 films, totalling 40 discs) will get you about £30, if you are very, very lucky.
And that's if their system even recognises the discs you are trying to sell them!

If the item is too new, or too old, it comes up as unregnised, and they won't take it.
If you can, sell via the AV Forums, or e-Bay. However, this is only good, if you don't need the money in a flash!
Pooch