Shorting bitcoins used to be very profitable on Plus500 because your positions expired monthly instead of daily now since the China explosion.
They also removed the leverage 1;10 so these two things makes it less interesting to short bitocins at Plus500.
The best and only alternative at the moment is to trade Bitcoin with leverage at AvaTrade. Your positions with AvaTrade do not expiry daily so that will give you a better opportunity to profit from short and long positions.
This topic attracted me because I actually own a website revolving around Shorting Bitcoins. You find it in my signature
Good luck.