I wouldn't recommend buying autographs, and would add that John's autograph has changed over the years.
There are loads of bad genuine ones about, as well as good forgeries with ''certificates'', so it's a real minefield.
I have some I got from John directly in '76 and '84 and they look quite different.
I can post them here if you want to use them as a guide, but ultimately you are taking a gamble unless you get one yourself.
Don't bother to ask anyone on here to authenticate either, they are simply guessing.