Thirty years ago, I suppose that the center of Washington's used book trade must have been Georgetown. Booked Up was near the post office on 30th St. NW, Yesterday's Books was on P just west of Wisconsin, Columbia Books for a while was above a movie theater on Wisconsin, and Georgetown Books was on Dumbarton St. And there were other stores with names I've forgotten--one that looked almost onto Key Bridge from the north side of M St., another downhill of Booked Up across M. For a while after that, Bethesda may have had the lead, with a Second Story Books location on Bethesda Ave. and Georgetown Books relocated to Woodmont..
Now the center may be a a triangle west and north of Dupont Circle. The Second Story Books at 20th and P has been there for all my years in Washington. Books for America has a store on 22nd St. just south of P. Kulturas Books on Connecticut Ave. just down from Florida is worth a look.
I found Books for America as a place to give books away, for the Washington, DC, library system no longer takes contributions of used books, having shut down its own store. But of course it is hard to walk into a book store and not leave with something. My last contribution to Books for America led to my purchasing one book there and three more at Second Story.