The vast majority of my guns were private party sales and I don't have receipts for the ones I bought from stores either.
OTOH, I don't keep receipts for anything I buy, and this is no different.
But, they are still your guns, aren't they? Why the paranoia? Saying someone seeing the gun serial numbers and reporting them being theirs and were stolen is just the most ridiculous thing I hear.
If your guns were totally legal, then who will be in hot water? The crook who report them stolen, or you? Why would anyone be dumb enough to do a false reporting. Reporting that something was stolen from you doesn't make it so. Got to have proof. I have yet to hear of a single case where the crook was able to pull one off and came out on top.
If your guns were truly stolen properties, then wouldn't it be the right thing to turn them in? I know I wouldn't want to have anything to do with them.