If you are going to accuse somebody of being involved in terrorist organizations then you better have proof as that is a very serious accusation.
Anonymous people barking at other anonymous people don't need anything other than time and dedication to remaining anonymous. Once you find out someone is trying to be anonymous, just stop communicating with them.
Bitscalper could technically sue you for a whole bunch of stuff (deformation of character etc) and get much more than just 1000BTC if you made that statement without any proof.
He could not actually. Even if he was dumb enough to out himself just to make a lawsuit (which would in turn incriminate himself for numerous reasons), BitScalper is not an identity, it's an anonymous site. It is not protected by any laws afaik.