For several years, I've purchased used books from garage, estate, and library sales for a dollar or less. After reading them, rather than allowing them to collect dust or clutter the house, I sell them on Ebay, Half.com, Amazon.com or through my own garage sale. For the books that don't sell, I would send them back to the library as a donation and take the tax deduction at the end of the year. This worked well, but I found that my book collection was building up in boxes while the books were waiting for new owners. At Ebay, Half.com, and Amazon.com, I would need to wait until someone contacted me to buy the books.
Last week, I found a new site called http://www.cash4books.net/. It is as easy to use as Amazon.com and Ebay. Simply register for free, enter the ISBN (number found on the back of the book), and see if there is a buyer for the book. If there is, you will see how much the company is willing to pay. Just affix the free postage label and pop it in the mail. I made $23 in around 25 minutes by sorting through all of my books. They will not take all of your books. In 7 out of 8 cases, my books were too old to take, but the process was so easy I was able to enter in over 100 ISBNs within 25 minutes to see the ones that they would take.
On all of these sites, your books should be in good condition so while reading them, do not fold over pages or overtax the spine of the books.