Here are 2 resources to sell books online:
Blue Rectangle will buy used books and textbooks. On their website, log in the ISBN number to see if it is a book they will take. If so, they pay for shipping with their postage-paid mailing label.
BOOKS INTO CASH
Books into Cash has been selling textbooks for 2 decades; They offer free shipping and no registration is required.
Think Outside The Bin
Think Outside The Bin 