Today I'm going to take a big stack of books to Second Story Books for trade value. Located within walking distance of the Dupont Circle Metro Station at 2000 P Street, they offer a good selection of used books of all kinds.
If you don't find what you want, and can't wait, Kramerbooks, also within walking distance at 1517 Connecticut Ave, may be able to help you. Open all night on the weekend, they also provide indoor and outdoor cafe seating with real food not just coffee drinks. Take a look at their online menu. (Try not to look at their Web design, though.)
Politics & Prose, not as easy to get to, but worth the trip, is another option for books in the Washington, DC area. Further up the red line, or on the L4 bus route, this bookstore at 5015 Connecticut Ave, frequently hosts well-known author events.
Support small, local and independent!