[panarky]

People said shorting Bitcoin was a no-brainer when the exchange rate was $2. When the exchange rate rocketed to $10, many serious, smart people just knew it was a speculative bubble.

Rinse and repeat for each leg up ... $32, $90, $240, $1100. Same arguments every time.

Given historical volatility, I'd say fair collateral for a short sale would be 300% of the current exchange rate in USD, EUR, CHF or gold, with a margin interest rate of 6% per month.

[mathattack]

The same argument held on Amazon, as the shorts got crushed on it's rise. :-)