Excellent sites for new and used books include:
Christianbook - find books, e-books, Bibles, music, pastoral resources, and more.
FetchBook - compare 145 book stores, 60 000 sellers for new and used books in a click.
BooksPrice.com - compare book prices and save money on books and textbooks; you can also sell your books through this site.
AddAll - search and compare prices for print books, used books, magazines, music, movies, and e-books (many of these are free to read online or download on your device).