One of my most favorite is Spies Like Us. It's a place for real Private Investigators, Cops, Attorneys, Secret Agents, etc. but 007 types like myself find it "velly intalesting .... velly intalesting indeed" (OK, it's a lousy German accent, but a great site).
In case you missed it on the index page ... this link is the place for small collectors (like me), comicbook storeowners, bigtime dealers, littletime dealers, and distributors - anyone who would like to buy, sell, and trade worldwide, on the Internet.
So if you have one comic, or tunza them, to sell - or, if you're looking for a particular book or series, please, take a look.