Most people think in terms of the local second hand bookshop or eBay, but if you're looking to sell books (or, for that matter, DVD's, video games, electronics and a host of other 'stuff'), you may do better selling on Amazon. It's a lot easier than eBay and you could be pleasantly surprised by the price you get - to the point that it can even be worth buying old books locally and reselling them on Amazon as a small business in its own right.
Just go to
www.amazon.co.uk and click 'sell your stuff' on the left hand column under 'Amazon Services'.