Ah, Vintage Hunter....., you have just identified the difference between the PRICE of something, the WORTH of something and the VALUE of something.
What you are suggesting is a most laudable and benevolent idea but........ and we've all been there....., where you agree a between-pals price for something and a while later, he, being shrewd (or a shyster) as opposed to benevolent sells it on auction to the highest bidder.
Ask me about the bundle of new, still wrapped No4 Mk1 cruciform (yes, real ones.....) spike bayonets I had one day