Hello,
The only way a comic can get our Signature Series authentication is if the book is actually signed in front of a CGC witness, employee, or through a CGC facilitator, and we immediately accept the comics for grading. That way we know with 100% certainty that the signature is authentic. If you don’t have a witness at the time of the book getting signed, unfortunately we would not be able to authenticate the signature.
When CGC is present at a Comic Con, we will have witnesses available at the CGC booth that will walk with you to witness a signature. Go to the CGC booth before getting a signature and ask to have a witness escort you.
We have witnesses available at multiple Comic Cons throughout the year to help you get your books signed and witnessed properly. You can find a complete list of the comic cons we will be attending at http://www.cgccomics.com/events
We do not accept COA's as proof of signature as they are easily forged. We would recommend that you keep possession of your COA as we do not encapsulate them with your book.
Your signed comics, if graded, would get our green Qualified label with a notation of "Name written on cover in marker" with no assertion of who may have signed the comic. By giving your comic our Qualified label, we are ignoring the signature for purposes of the grade.
Please let me know if I may assist you with anything further.