By now, it shouldn't be too surprising that I love bookstores. I'm a total skinflint about everything else, but I love buying books. (I love libraries, too, but it's so nice to add a book to my personal library!) So used bookstores are great--books at reduced prices? Heck, yeah!
John Rossheim lists a few of his favorite used bookstores. New England folks will recognize Raven Used Books in Cambridge and Myopic Books in Providence. I would also add the Brattle Bookshop, and the basements of Brookline Booksmith and the Harvard Square Bookstore.
As for selling books back, here's a way to get rid of those novels you had to buy for your 20th Century Modern Literature class:
"There's always a need for classics like Faulkner, provided they're in reasonably good shape," says Brendan Sherar, (founder and CEO of Biblio.com, which sells used and new volumes from thousands of independent bookstores).
I guess I can get rid of a few of my multiple To Kill a Mockingbird copies...but what if I need another?
(I'm only hoarder about books, I swear.)