A great web site for rare and technical tomes is http://www.abebooks.com/
I list my rare and otherwise specialized books there.
I cannot say enough about ebay either. You can often find buyers there that you can't elsewhere.
In either case, list those goodies about 6 weeks before the semester starts. You never know who may be collecting, or a prof is going back to a favored text.
Good luck and Happy Easter!