Unlike futures (say a 2 year crude oil contract), you can only short bitcoin for day-trading. The trick is that in most exchanges you pay a huge fee everyday to hold your position. So you can't say, "I think Bitcoin will crash in 2018" and open that position. In a few days the fees eat up everything and you are forced to liquidate at the price that day. (see bitfinex, whaleclub, etc, etc)