I could only think of three aside from AD:
Namepros.com
DigitalPoint.com
*******.com
But at DNF, you have to be a platinum member first before you can sell and buy domains. That equates to $40. Not as active as NP and DP but if you want really "serious" buyers with
huge budget, you can find many here. Just make sure your name has that huge following in the search engines and belongs to the "endangered" species club.
I don't recommend to sell your domains at DP, though, unless you're willing to get paid or sell your domains 20x less than their real value.
Namepros is viable for all seasons.
I just found out about AcornDomains recently, actually, that's why I registered. No doubt it's Number 1 in the UK market.

I'm thinking of expanding my portfolio.