Outside of brokerages, I think the best venues to sell a boat is a local one.. This forum is in fact well represented world-wide.. but I suppose that fact also thins out the available buyers for any given listing. Also I don't get the impression that the majority of members here are 'in the hunt' so to speak, and few are actually trying to sell at any given time.
Another factor is that this particular forum is intended for long time contributing members of Sailnet, it's not meant to be a 'drive-by' bulletin board or a general classified type listing like Craigslist.
Long distance buying is not uncommon, but I think generally you'll have more success listing on a local venue like CL where the buyers can get a quick personal look easily with minimal travel. An exception may be when a buyer has locked onto a particular design and then starts hunting for one of those.. then Yachtworld listings (brokers only, though) and the various craigslist boards come into play more than this forum might.
In general, though, I agree it's under utilized...
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)