I've dealt with some auction houses that require the gun to go through a FFL because they can not definitively prove that the gun was made prior to 1898 due an absence of records that have disappeared as has happened to many English makers.

I can't blame an individual seller or auction house that chooses to CYA rather then risk having a chat with local or Federal law enforcement.

I have become addicted to English hammered shotguns to the detriment of my wallet.