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There is really no good shorting trade on these stuff. Even if you believe perfectly in its eventual demise. It might run to 50,000 first. Suppose you shorted at 10,000, with $10K, meaning one bitcoin. When it goes to 50,000, you have to put up 40,000 to meet the margin requirement, so if it eventually goes to 0, all you make is 10K on a 50K outlay - or 20%. If that takes 5 years, that comes to just 4% annually. For a high-risk trader, that is nothing - useless tying up of capital.
And if you think options is the way, again it is too lop-sided. On Dec 20, I took a screen shot of the option pricing for RIOT, March 2018 expiration (attached). Stock was trading at $35. The Mar 2018 35 PUT was trading for $18 (while the CALL was $4.00). Such disparity that shows every intelligent being knew it was going down big time. Unfortunately, if you bought the put, and this goes to $10 by then, all you would make would be 50% on the option. Too small for such a trade for me.