Ukazoo Books in Towson. A stack of my gently-used hardback books brought $8 cash or $9 store credit... so I used the store credit and left with a couple of used audio books. Score!
You can peruse the shelves while you're waiting for the Book Buyback clerk to process your books, or you can choose the auto-accept option and leave with a receipt for the books they decide to buy. The unsold books you leave behind will be recycled into paper products.
Let me sum up: unclutter your bookshelves AND cash in or trade for better books. What's not to like about that?
Alternate options: www.bookthing.org and www.paperbackswap.com as mentioned in an earlier blog post.