Being able to sort by distance like on eBay would meet this criteria. Maybe a local pick up option or check box that you could sort by would also be good. I also like the feedback system for buyers and sellers so you can see history and experience with past transactions.I know at one Time the app asked for location to help network with people close. If there was some way to have a seller forum for people within a certain mile radius. That way, for some the price of corals along with shipping makes it a little prohibitive, if they can keep it in their vicinity they could probably eliminate the shipping charges by meeting some place.
Edit: like a Craigslist for Frags. You could call it Fragslist. LOl
I would still personally unto shop some of the Nationwide Network, but to have a local Access Network might help bring in more people into the coral Market that otherwise couldn't afford to buy the corals after you add in shipping.