Amazon.com - http://www.amazon.com
Amazon is the premiere website for selling back your new or used textbooks. It has an incredibly wide selection of books and academic materials, so you will have little trouble finding listings for your textbook-even for the strangest and most obscure texts on the planet, and even if they aren't in English. The site has the highest volume of marketplace buyers around, so your book will have a good chance of selling, and selling quickly. Money is distributed directly into your bank account after a few business days, and the buyer pays all shipping costs.
The only drawback with Amazon is that it offers marketplace only-no instant buyback, but the price that you will receive for your textbook via the marketplace will more than make up for it.
Unless you need instant money back for your textbooks, Amazon is the place to start.
Valore Books - http://www.valorebooks.com/
Valore is one of the lesser-known online bookstores, but it is comparable to some of the major online retailers. It features instant buyback for selected textbooks, and provides instant quotes for the books you sell. They guarantee to send your payment within 48 hours, and even cover shipping costs. And with over 500,000 sellable textbooks, you have a good chance a decent shot at getting a quote for your book. If not, you can give the site's marketplace a try.
If you can't find another retailer to accept your book, Valore Books is definitely worth a look.
BookByte - http://www.bookbyte.com/Default.aspx
Book Byte offers many of the options found with other online booksellers-instant quotes, free shipping, rush payment-but with an added dimension.
Book Byte's beauty is in its versatility. You can sell back international editions of books, reference books, K-12 books, and offers services for volume sellers. It even has a customer service line hotline dedicated to fulfilling your every need. It even has a student marketplace so you can sell directly to other students.
If you're looking to sell a unique book that isn't accepted by the other sites, Book Byte is a good last resort.
ECampus - http://www.ecampus.com/
ECampus is a great resource if you want to get a good price back for your textbook. The prices books sell for on the marketplace are considerably higher than on Amazon, but as it has far less customer volume, you will have to wait significantly longer for your book to sell. It offers an instant buyback option, with shipping costs taken care of and a wide range of accepted books, but they only accept a limited number of books and even then the given quotes are extremely low.
The major drawback with ECampus is that they only pay by check, and they take their time disbursing the checks-sometimes longer than a month. So if you're looking for payment sometime in the next week or two, skip ECampus completely.
Blue Rectangle - http://www.bluerectangle.com/
Blue Rectangle is a low volume online bookstore that doesn't offer a marketplace. It does have an instant buyback program, but the volume of books doesn't seem to be very great and only lets you search by ISBN, not title or author. The site pays for shipping costs with a prepaid label, and its search function allows you to get quotes on multiple books at a time.
The only thing unique about Blue Rectangle is its payment program, which allows you to choose from being paid by check, store credit, or by Paypal.