Shorting Bitcoins would be a terrible idea. I hate Bitcoin, and expect it to eventually crash, but if you nakedly short something and it goes up in the meantime (which BTC could) you can get whacked with margin calls and lose money even if you're right. That happened to a lot of people who shorted the Nasdaq (at, say, 3000) and tech stocks in the '90s. They were right, but they still lost their shirts.