[COFF] Marketplaces for Vintage Tech Books, Etc.?

Good afternoon folks, I just wanted to ask if anyone is aware of online marketplaces I should be looking at in my constant scouring for historical documentation materials? Presently I've got a policy of checking eBay and Biblio pretty regularly for UNIX material, occasionally searching for a few other odds and ends subject-wise, but I'm starting to wonder if there are other avenues flying under my radar where folks might be more likely to be selling for instance 70s and 80s UNIX manuals, paper copies of old standards, hardware docs from IBM and DEC, etc. If you have any suggestions, especially those that don't require me to setup yet another account to keep track of, I'd surely appreciate it. Also consider this my way of saying if you​ have something to sell, I'll gladly consider it, although I am being pretty selective on matters of historical/research significance that are currently obscure, so sorry if I won't buy your twelfth copy of KnR C, even if it is signed! - Matt G.