Is there even a US based legit platform to short Bitcoin? If Coinbase was serious about having a true market they would allow you to short Bitcoin as well as buy.
Coinbase does have a market tho, it's called GDAX, you use your coinbase login and it has margin trading on it. Once I found out about GDAX I never used coinbase again for trading currencies because GDAX has no transaction fees if you are creating the transaction ie. setting up the buy/sell limits