Apple need to create a system where stolen phones can be reported to them, Apple can then contact the owner/verify they are stolen. And then add them to a stolen list and disable calling/apps on those phones. And display an overlay on the screen THIS PHONE IS STOLEN.
Every iphone would come with an validate phone feature that is accessible even when locked that can authenticate the iPhone for anyone thinking of buying it.
The potential buyer can check if the iPhone is stolen by using the feature that is allowed to connect to the internet and validate the phone.
They need to make it where stolen iPhones are worthless so when you are getting mugged criminals won't even want it.
Obviously have an option setup where you can transfer ownership of your phone. Maybe with a 7 day waiting period.