This could end very badly for Bitstamp. If the buying slows down enough from most of the traders on Bitstamp exhausting their supply of dollars, Mr. 30k might keep reducing his ask price until he has sold through most of the order book. That would leave everyone else on Bitstamp with lots of bitcoin and only enough dollars to support a far lower price (probably under $100). At that point, people would rush to move their bitcoins to a different exchange to sell them. The only things that would prevent this from happening are a lot of dollars that aren't currently being used for any orders or the 20k wall being bought very quickly or pulled.