My understanding is there are no "proof laws" in the U.S..

That being said, clearly the proof status of English guns (and likely other foreign sourced guns) has a bearing on value.

Would I buy a foreign gun that was out of proof? Possibly. Just identifying whether it is out of proof by measurements would be the first step. Taking into account some loss of value due to that and the physical discrepancies that make it so, along with the effect on performance /strength, price, potential resale issues, desirability, etc, would all play into whether I'd buy it.