The Harvest Book Company is located on State Street in Media. It has a large selection of used books. Books are in very good condition and all adult titles are $2!! I recently found a hardback mystery that was on the best sellers list. I love browsing their new acquisitions. The titles are organized very well.
I don't know the details of their buyback program, but I have witnessed folks turn in books for credit against their next purchase.
The best deal here, though, is the Children's section in the rear. That's where you can find very gently used books (often they haven't been cracked) for one dollar. I just purchased a brand new hardback scholastic book of presidents for my kids. A great reference to have around the house.
Call first before visiting, their hours are quirky.