I've used PM sent when someone is looking for help with one of our machines, to let others in the thread know, that this individual is getting support from us so no one else needs to be throwing out suggestions on how to solve their issue.
So reading this thread, a "PM sent" in a "need help" scenario is clearly different than in sales thread, where like others, I suspect it is more often than not a lame attempt to scare off other would be buyers. It probably does scare off a few, but doubt it is really that effective.