I would never buy from such a poor small image. I would never buy anything framed unless it was opened and properly scanned for me, and/or understood and in writing that I can open it for examination without losing my return privilege.
It is not "especially rare" as an image. At 16x20 it is likely a copy print anyway. The signature cannot be seen well enough to evaluate sensibly.
The certificate is 25 years old. A+ Sports is, or was, just a UACC member, but not a Registered Dealer. The UACC does not endorse the use of certificates.
It is framed and I doubt you will be allowed to open it. and return it.