Which Exchanges Allow Short Selling/Margin Trades with USD/BTC or USDT/BTC I think Bitfinex did. But for example Poloniex has USDT &amp; BTC but does not allow short selling with USDT.
There's apparently a website that reviews various exchanges that allow trading on margin:

http://margintradingbitcoin.com/

OKCoin and Bitstamp are the two that I've heard of on there.