Where to Sell Used Books
It starts at Books for Cash, and ends at Re:Books for Cash.
The basics are:
"Ethical used bookstores pay between 25 - 30% of their selling price (which is, in turn, generally half the current retail price) in cash. Trade is generally 50% of sale price."
They cover Elliot Bay Book Company, Third Place, Twice Sold Tales Capitol Hill and Queen Anne, Twice Sold Tales U District, Magus, University Bookstore, and Spine and Crown.