Cracked Back and Front Screens

The entire phone will have to be replaced.

$399 in the US.

The internals are laminated to the back glass. If the back glass is cracked or broken, the phone is not considered repairable at all.

Apple will not replace the display if the back is cracked.

He's absolutely correct. If the back glass is broken, the phone cannot be repaired and will required a full replacement. You can purchase an out of warranty replacement for $399, or buy an entire new phone. Your phone has no value in trade whatsoever.