Bitfinex is the place to go.
Avoid BTC.SX it's basically a fraud lol
You gotta know what you are doing though, plan your entries carefully and always set stop losses in case the price goes against you.
Of course you can go long as you can go short.
If you just looked at the graph and said: "cool this will go down i will easily get rich shorting this" I would strongly advise against it, because if you don't plan anything and you don't have much (margin) trading experience it will probably end in tears.
How are they a fraud? Never had issues with then before?
Fees are astronomical and there is a murderous spread.
Technically the site is "legit", yes, but it's a mess.