If it's easy to buy subprime mortgages with cash does that imply it's just as easy to sell said subprime mortgages for cash? Substitute any illiquid asset(relative to U.S. dollars) if you wish and the statement holds true.

I think the asymmetry revolves around the ease/difficulty in getting USD in and out of an exchange like mtgox. You can take physical paper money to the bank and transfer to an exchange, no problem. You can buy BTC at the exchange, no problem. You can sell your BTC at the exchange, no problem. You can transfer USD from the exchange to your bank and withdraw physical paper money, maybe a problem. Perhaps a slight delay.