There might not be a formal, publicly-accessible market, but two-party short-selling contracts could already be active. This is what an unregulated and anonymous currency feels like: you have no idea whether I have purchased half of the bitcoin market over the last few weeks just to unload all of it at once, in small chunks, before the market can adjust to the reduced value. In fact, you won't even know if that has already happened; you will just see a "market correction" when I sell.
