"at a market rate" doesn't mean "at a current price", but "at the amount people would be willing to pay". Selling such a high position would lower the price of

"at a market rate" doesn't mean "at a current price", but "at the amount people would be willing to pay". Selling such a high position would lower the price of bitcoin on all the exchanges, until enough people would be willing to buy them. So, for example, if you wanted to sell 100k BTC on MtGox in an instant, you could do that. It's just that the price would get down to around $120, and you'd get "just" $35M. (real numbers, via http://bitcoinity.org/markets/mtgox/USD http://bitcoinity.org/markets/mtgox/USD ) As for Coinbase - as far as I understand, they don't hold a bitcoin position, but use internal trading and external exchanges. So both the money and the btc would come from people willing to buy Bitcoin.