It's perfectly legal to sell an out-of-proof gun here in the US but I wouldn't touch it for (nearly) anything.

Having a new barrel fitted to a H&H (for example) by the maker might run 15-18KUSD. A second-hand H&H with original barrel might fetch 40K; with a maker's replacement barrel it might fetch 30K.

A London Best with a new barrel by the maker is a good deal in my mind - I would love to find a 1938 receiver with a 1990s barrel fitted by the maker!