So, the selling of an out of proof gun would be a tort, even though both parties agreed upon the sale? Who would be the plaintiff in this court action, and how would they have standing?

I'm fine with the proof house in existence in England, or wherever else it has been in effect since we were a colony. That's not the issue here, as you'll discover if you read the title of this thread. You guys also drive on the wrong side of the highway, unlike most of the civilized world. Just because it works for you doesn't mean it would work for us.