Had a great time browsing at Westsider Rare and Used Books
The space is exactly what a secondhand bookstore should feel like and the books come cheap (I got one for $4 and one for $5!). Definitely more work than a firsthand bookstore but half the fun is perusing and getting hidden gems. Also near Central Park so can go read the books you got amidst beautiful scenery.

Westsider Rare & Used Books Inc.
New York