From the data that we have this is how the market looks like:
Undeveloped: 2,700,000 domains listed of which 710,000 parked
Sedo: 19,000,000 domains listed of which about 5,000,000 parked directly and a large number of names forwarded from other parking providers or home made landers (we can't track those).
Afternic: 13,000,000 domains listed of which 240,000 parked.
In terms of exposure, I think you're referring to the distribution engine that both Sedo and Afternic have. Since both companies started that model years ago it's impossible for new entrants like Undeveloped to compete on that front. Distribution parties have already integrated for example Sedo MLS and aren't keen to do a new integration as it's quite a headache to implement MLS.
Now Afternic is only performing so well in terms of sales because they are acquired by Godaddy and ever since that acquisition happened Godaddy kicked out all other marketplaces and only allows Afternics inventory to be promoted in Godaddy's search. As you know Godaddy controls a large piece of the market so that's why listing at Afternic is a good idea.
At undeveloped, we're working on an open domain pool to democratize access and promotion of secondary market domains to all instead of building closed networks like Sedo and Afternic. You can learn more about the Domain Pool here:
We invite everyone to join this open network to increase liquidity in our market for all. So that's our attempt to get in that market but with an open model in mind.
I always advice, all our sellers to list their names on any marketplace they'd like to work with. We don't demand any exclusivity as it's our goal to get you more sales not just to increase our own bottom line. But the best setup is to use our for sale pages in combination with leveraging the distribution engines that other marketplaces have in place until the Domain Pool is live and adopted as the standard network.