You could try green metropolis
http://www.greenmetropolis.com/
Click on Help and it tells you how to do it . It's easy to use .
Doesn't cost you anything to list them .
By the time you weigh up listing costs from some sites .eg EBay ,the hassle of posting them to people,packaging etc you may be better off trying a good secondhand bookshop. They may offer you a job lot price depending on the genre ,condition etc .Or you could do a boot sale or if they are really nice books of specialist interest an antiques fair type thingy :)