Lens & Repro is only open weekdays, but well worth the visit if you can make it in. Adorama and B&H are both worth a visit for the used department - lately I've been seeing some real bargains at Adorama for some reason. B&H is an absolute cultural experience, a must-see. If you're there during the week, you could also try Eli Kurland (mostly high-end stuff like Leica and Hasselblad). There's also Fotocare which is near Adorama, and Calumet (although I don't know what their used department is like these days).