Helen - I've never found that books sell particularly well on E-bay, but I've had considerably more luck through registering as an Amazon marketplace seller. You won't get rich this way, but it is a good way of getting at least something back for books you no longer want.
Basically you fix your price, and then the book is listed for IIRC 60 days. If somebody buys the book in that time, you get the price paid plus 2.75 P&P, minus Amazon's not insignificant fees automatically paid into your bank account. All that remains then, is to print off a shipping label, and post the book off to the buyer.
I tend to list books once only. If they don't sell in that time, then they go to a charity shop.