Raspberry Pi is actively preventing enthusiasts from upgrading soldered memory to a larger configuration even when the replacement chip is technically compatible. The company says the restriction is intended to discourage unofficial high-capacity resales.
Google have an imei checker for their phones. Google.
You could check that it is real and active.
Raspberry pi could allow people to check their board model on their site to see if they are getting the right amount of RAM. They could devise a method to allow checking the correct RAM is attached and not a recovered stick off another board of the same size.