Just a quick recommendation for a website today. Greenmetropolis.com is a site for buying and selling second hand books, all of which are priced at £3.75 each. The postage on an average paperback is free but on heavier books the seller can add a postage cost.
I have bought a couple of books from this site and they are always in good condition.
Shameless plug alert - I have just listed "Stitch'n'Bitch Crochet, the Happy Hooker" on the site. It is in perfect condition as I never used it, just couldn't get my head around crochet! (Edit: book has sold)