can someone recommend a good exchange where I can hold US dollars and short BTC with it? seems to be difficult to do for some reason.
Margin trading is available on Poloniex and you can hold US dollars there. I personally have not made any margin trades so I can't say for sure whether their platform for those kinds of trades is user friendly or not. However, I do other trading on that site and it seems to work for me and it has a pretty good reputation.
There will be loans for short sales and the rates will vary based on what rates other customers on the site are willing to lend their bitcoins or other cryptocurrencies for so you may get a low percentage loan one day but a high one the next. Also, you may need to fund your account with Bitcoins and then convert to US dollars.
