I don't think anybody with such a large amounts of Bitcoins will try to sell them at once. Maybe in the future when some big businesses start accepting Bitcoin and then from time to time they will exchange there Bitcoins for dollars for example, but they the will buy some Bitcoins back in my opinion.
why not, if he declare is income and everything is transparent, i don't see any problem in selling large amount of bitcoin, you need verification on any exchange for that amount, so they have everything to track you if needed
or maybe you want to sell to buy big later, so it's just a trading activity in this case ti should be even more easy to do, because the money will never leave the exchange