You can go to any of the major bitcoin markets and sell thousands of dollars of coins while hardly moving the price for USD. E.g. on MTGOX right now the best b

You can go to any of the major bitcoin markets and sell thousands of dollars of coins while hardly moving the price for USD. E.g. on MTGOX right now the best bid is $271 and a market order to sell 100 bitcoins right now would net 27056.5448 USD and would take the last price down to 270.1650 USD, resulting in an average price of 270.5654 USD/BTC. Selling 1000 BTC would result in an average price of $266.9482. Its certainly not the most liquid thing and getting funds to and from exchanges (or buying other things) isn't as available as one would hope, but it's not highly illiquid by any means. There are good markets for other major currencies too, though none are quite as liquid as the USD ones.