As you say Simon a COA is worthless, and I don't care who it's off.
I have several hundred Autographs I collected from being a Kid. The only ones I've bought were off a friend of mine, we both collected together starting at school, being like minded Monster Kids.... We would get them through the post, as "celebrities" were a bit more accommodating back then, well most of them.. Now-a-days, things are so different.
In recent years I've had items signed, as I've met quite a lot of the people involved with films I like, you get to meet them and watch them sign your item.
To "buy" any signature is a bit of a minefield, which is always open to fakes, more than any another field I think. The problem is if you want a person' signature who is dead, you have to tread that minefield, and this is where research, and more research is vital. It's not for everyone, but then again neither is collecting dusty old posters....