sorry for late reply, I use a few sites.
You might be able to use them depending on your location
markets.com trade.com plus500.com
They are actually for REAL MARKET TRADING but also have CRYPTO SECTION
you cant trade all cryptos but you do have enough choice to make cash from them
Poloniex works. Other ones include Bitfinex (depending on location). Or, Bitshares offers a decentralized exchange that allows for margin trading.
Edit: well, short one crypto versus another.
sorry for late reply, I use a few sites.
You might be able to use them depending on your location
markets.com trade.com plus500.com
They are actually for REAL MARKET TRADING but also have CRYPTO SECTION
you cant trade all cryptos but you do have enough choice to make cash from them