Ditko has for a very, very long time refused to sign comic books. He wasn’t interested in the past and as follower of Ayn Rand's Objectivism philosophy, he see's no reason why he should do anything he doesn't want to do. He was known to respond to letters, which he signed.
Any comic book with Ditko's signature is almost certainly forged. He wouldn’t sign comics. Not for a charity, not for a friend, not for the kid with cancer or any other story you are told about the comic you found for sale. I wouldn't even fully trust VSP on Ditko's signature.