Do I trust anyone outright on eBay? God no. I just like being able to learn a bit more about the person. If a seller has a good track record and says they are a tPFer, I ask them for the tPF screen name and literally see what they talk about on here.
I almost bought from a tPFer who seemed active in Chanel and had a Bonanza store. However, and I was honest with her, a basic google search pulled up a load of drama and well, it made me unsure. That is the benefit of knowing if someone is a tPFer to me, I can get more info on who I'm dealing with. Its not fool-proof, but I'd have that over nothing.