Have you ever considered using the Bitsquare platform as a domain name exchange?
Check out Sedo.com, Afternic.com, Namejet.com, etc.
They charge 15-20% for domains and their systems are centralized. This would be an amazing service and you can build in the Escrow.com API or your own.
You would allow individuals to sell domains directly instead of going through centralized expensive exchanges.
Perhaps in the future Bitsquare becomes a market not just an exchange. Bitsquare is still in it’s early days, but we might see this happen one day. Either way, I doubt domain name sales implementation will be separated from the implementation of the rest of the markets.
I would imagine OpenBazaar would be a better decentralized marketplace for domain selling. It wouldn’t surprise me if it’s already happening
Now, if we could just get that elusive domain name system 
I noticed github commits for namecoin were very high compared to a lot of top 20 cryptos the other day. Perhaps that project hasn’t drawn it’s last breath and I know there are other attempts being made.
I think Tor network made a good system for domain names of hidden services. Simple, private and secure. Derived from hashes of a public key and no site exists without a public key, that is maybe the future.
Yes, HS are fantastic! mapping readable name to the onion might be a useful service, prob. there are but i am not aware of it.