Someone (I think on this forum but I'm not sure) made a post about bringing a character from the opposite faction to hellfire to get easy marks. They were contacted by a GM very quickly. I think they were only given a warning.

Having a character /who'ing people, we had someone in the zerg doing it for a while without any problems. It does get close to harassment but I think as long as you don't stalk someone 24/7 you should be ok.

Trading faction specific stuff is fine.