Try posting your inquiry on a local newsgroup or just google for "used
books Cambridge, MA" and see what turns up. Good luck.
What do you mean by "very selective"? No book store needs to buy stuff
they can't sell. Classics? Yes. The 47th copy of a book that was on the
NY Times best seller list last year that no one can even remember today?
No. Romance novels? Forget it - they aren't even worth trying to sell on
eBay. No trade store wants text books and computer books. And so it
goes...
Missing from your list are:
Brookline BookSmith
Boston Book Annex (Brookline & J.P. locations)
Rick