Hey Greg
If it was me and I was 100% sure everything was legal with the sale I would " find " the receipt. But I would put the seller down as a good mate whom I had known for over the 15 years. The mate who you were with when HE bought the car 15 years ago and then sold onto you at a later date. Of course you'r mate lost his receipt years ago
. Also I wouldn't offer up the receipt unless asked for it.
It's probably not legal and I'm not suggesting that you break the law but It's what I would most likely do in the same circumstances with no other options.
Brad.