Sample authentic letter a buyer got seller to sign with a purchase

[baalton]

Does any anyone have a sample of a letter or note they got a seller of an item to sign. A game used signed item that has no psa or any coa. The seller was give the item, totally legit and 100% authentic. I would like to draft up a short letter or note with a few details I know and have the seller sign it. Just to have it, have the story behind how the seller received the item and to have it for future potential buyers way down the road.

Not like a legal lawyer letter, they sometimes do this on American Pickers TV show. I looked online and no luck.

Any samples, wording that should be included or any hints or tips.

Thanks