Granted the market is easy to manipulate with small transactions that can start large swings; as well it would be difficult to find a buyer if you wanted to liquidate a lump sum of this size without a solid discount.

That said, I think you're overstating the impact this would have. The recent downswing from ~$30 to ~$10 in a period of a couple days was speculated to be due to MtGox consolidating 20 times this amount.