For selling books, I highly recommend Kensington Row Bookshop (3786 Howard Ave, Kensington, MD 20895; 301 949 9416; krbemail77@...) and Capitol Hill Books (657 C St SE, Washington DC, 20003; 202 544 1621; info@...), both of which actually pay fairly for books (by today’s standards) or, alternatively, offer generous store credit.
Kensington Row Bookshop has great parking; Capitol Hill Books not so much.
Lest you have a large collection of books (a tall bookcase-full or more), they will not come to your home, however – unless the books are exceptional.
I would suggest taking photos of the books as they are in the bookcase(s) – all of them, if not the very best of them, – counting the approximate number, and calling these shops with a brief list of your best titles in each category, to give the buyer an idea of what you have. They may or may not ask for the photos but they’re always helpful to show condition.
Final point: Mention dust jackets, if your books have them, as they matter. Mention condition, as it matters too.