From their web-site:
So, since I have pretty much all of those "regular" comics, I'm not looking for all the stuff that is in AA's "sweet spot". The 60s stuff I'm looking for can be found lots of other places for better prices than the premium full guide value that AA members tend to ask for this stuff.Suddenly, we thought to ourselves, “You know, there are tens of millions of comics that have been entered into ComicBase—all with prices. What if we just gave everyone a big button marked “Sell” which would let them post their comics to a central site. Then, anyone looking for a regular comic like Hellblazer #85 could not only find it, but probably find a dozen copies in various conditions. And whoever sells their comics on the system would be able to put their books before the entire world and just wait for orders to roll off their printer!”