What does it mean when a new iPhone is not registered?

Hi, an acquaintance of mine just bought a new iPhone and due to some personal reason, he's planning to sell that phone which is new (unopened).

He has the bill and also gave me the serial number of the iPhone. When I checked it on Apple's website (Check Your Service and Support Coverage - Apple Support), it says that the device is not registered.

I'm not sure what this means, because from my understanding, all iPhones are registered with Apple, and I was hoping someone would help me figure out if this is a legitimate purchase or not.

Thank you.