If you base it on the App Store account that handles both cases pretty well from Apple’s perspective.

US users can buy all the EU phones they want, with a US account you’re treated like every other American.

EU users can buy a phone anywhere and since they have an EU store account they get side loading.

Where the phone was purchased from is irrelevant. All that matters is the verified payment address.

IIRC this (iCloud account address) is how Apple decides whether or not you are a Chinese user for purposes of Chinese regulations.