[SwiftDeath]

The word you are looking for is provenance

Basically you want to be able to prove the provenance of the piece without a doubt

If possible I would say you really want a personalized & signed letter from the original artist as that would easily hold the most weight to it

I would also get a signed letter from the seller but that is less crucial in my mind

Also if they can make the letters on official ubisoft quebec letterhead that would help significantly