The East Village Books buys and sells used books. They are located on Saint Marks Pl in NYC, NY. Their phone number is 212-477-8647. Also, the Book Ark buys, sells and trades books. They are located on W 81st St in NYC, NY. Their phone number is 212-787-3914.
Strand Bookstore is the first bookstore that comes to mind when thinking of Used Book Stores. They have two locations in NYC. Check them out at www.strandbooks.com. As far as N.J. goes, try Franklin Books in Somerset, NJ