I've got a Mickey Mantle autograph on an index card.
As you can see, it's
not signed in Sharpie. Index cards are my preferred item of choice because I like the signatures for their own sake, and Sharpies don't work well on 3x5 cards because they bleed, and even if they didn't the point is too wide for my tastes. When it comes to 3x5 cards, my writing implement of choice is a Pilot Precise Rolling Ball, extra fine point, in blue.
This is the only Mantle autograph that I can ever know to be genuine because I acquired it in person, he signed it in front of me. I can't say that about any other Mantle autograph, no matter how reliable and well-respected the authenticator. Hence, this is not for sale.
(For informational purposes, here's the background behind the autograph. I acquired it at the Mickey Mantle's Restaurant in New York City on July 16, 1991 during a press conference in which Mantle publicized an auction of his 1967 road jersey. When I gave him the card to sign he looked at the back, probably to make sure it wasn't an unlicensed item, then signed it and - at my request - dated it.)