I am an admin in a fairly large comic group on Facebook (Comic Book Lovers Buy, Sell, Trade, and Auctions). We have a core group of sellers that have been around for years that offer a wide range of books. I think Facebook is great for buying as opposed to eBay because of the interaction. It is more personal and I have made some great friendships from it. I think Facebook will take the marketplace feature to the next level eventually. When that happens it could be one of the go to spots for comics. It's definitely heading in that direction already.
BIGS