I wouldn't bother with Music Magpie - their prices are pence per standard CD.
Or E-Bay either - there are far too many people doing the same.
Your best bet is to either have a car boot sale, or pal up with someone else who is - and put them out at £1.50 each, and be prepared to take £1, and 50p for the last hour.
Other than that, check out your local second-hand shop - they usually do trade-in against new albums so that's a better deal as well.
If you hgave a seriously alrge collection, check Yellow Pages for any dealers who collect - they will buy in builk if you want them off your hands.
And as a last resort - send me a list - I may be able to take some off your hands - you have my e-mail? if not, I'll send it down.