Are your book shelves becoming overcrowded? Even the most dedicated book collector sometimes needs to get rid of the excess. Contact your local library to find out if they'd like your old books; they may be able to be sold at a book fair.
Another option is to register your book with BookCrossing.com. Just visit the site, register your book and print out a label to stick inside the book. Then, send it out into the world by leaving it on a park bench, donating it to charity or giving it to a friend. If someone comes across your book, they can visit the site and let you know where it ended up!