Hi Jackie - Another suggestion is Better World Books. You can sell through them, or donate to them. In my area, BWB has bins in a few shopping areas and you can just put books through the slot. Check their website, I know I have seen info about donating/selling - probably in their faq section. DH and I have always been happy with their prices, selection and service, and we have dropped off piles of books in their bins.
Lefty , in Philly