With a Fox or a Parker youre dealing with only the American market as most people overseas do not recognize our doubles as anything more than hardware store gun built for the masses. But, the American market is very good and the ease of transfer of guns in this country is a plus also. Concerning a Purdey or a Holland & Holland either one is recognized by the ultrawealthy in the world, but you're dealing with all the rules and regulations in foreign countries and you'll need help there.
My advise if youre looking to invest in guns then the same rules apply, buy low and sell high, which means one thing, find an original Winchester Model 21 overseas as they sell for a fraction of the cost over there, then bring it back here and sell it.
Good Luck no matter what you decide to do.