Has CBR ever considered allowing that? Selling and Trading of comics is a large part of Comics fandom. Not for actual negotiation, that should take place in private, but just someplace to allow people to post information about what they have or are looking for, or link to an external database site to allow people to browse their collection or wishlist. Someplace like comicbookrealm or similar.
You can check comicbookrealm.com which is an online database and price guide. You can enter you comics into their system and then sell from there. And it is free membership. You can pay to support the site, but it isn't required.