when selling: gox

But the magical $ value you see in your mt.Gox account after selling BTC is worth $0 effectively, if you cannot take delivery of it. (i.e. wire transfer to your own bank account)
Who cares if the other person isn't willing to demand use of a functional exchange for pricing?
Bitstamp for valuing when buying and in figuring out how much I have, Gox if anyone's enough of a sucker to take it when I'm selling.