ValoreBooks.com is extremely reliable compared to other online used textbook retailers.  They offer a toll-free customer service line (1-888-VALORE 1) whereby a real person answers the phone, and NOT some annoying phone system that makes it impossible to speak with a real person.

They offer an automatic 5% discount on all purchases year round with the use of coupon code: BIGFIVE and are quick to offer refunds or partial refunds if the customer is not completely satisfied.  Their system is easy to navigate and search for books by title, author, or ISBN #.

They have two actual brick and mortar locations in New York state where they sell and buy back used textbooks.  They also offer an additional 5% cash back and offer instant sales quotes on books you want to sell back with the same coupon code: BIGFIVE and also offer free shipping when shipping a book to them for buy back. 

They have saved me literally hundreds of dollars each semester, and I get my books virtually at NO COST when I sell back my old books and buy new ones with the proceeds.  I would highly recommend Valore Books.com