To put it in perspective, the daily volume in BTC is a few 100,000s. 5-600'000 BTC is $10B. Sure the Winklevoss twins can't liquidate their entire holding in a week but if you only have a few million dollars worth of bitcoin, you can liquidate in a few days without moving the price significantly.

The other major crypto-currencies are more or less liquid in proportion to their market cap.