If you want to make sure that thieves do not steal iPhones, you need to ensure there is no point in taking them apart for parts.

Which means you need to prove both: you are the owner of the device; and that the parts are not stolen. This means serial number on each part (likely already exists), database of individual parts with "stolen" flag, and a secure way to query this before pairing.

Or don't bother with all that, and just prohibit pairing of parts with unknown origin - which means prohibiting all 3rd party services.