Theft is a scourge no doubt.
I don't however, believe technology will be a quick way to stop it. If everybody in society would stop buying stolen merchandise, the market would go away. I will accept that some buyers of activation locked phones do not realize it may be stolen, but not all. Not by a bunch. There is a reason a $800 phone is only $150 when you buy it in an alley from an in-a-hurry seller.
Find My Phone exists to reduce the value of a lost/stolen phone to zero. Sadly, that hasn't eliminated the market yet. It is not Retrieve My Phone. Once stolen, your chances are remote you will get it back.
Your data however is safe. If your passcode can't be guessed, the thief can erase the phone and remove it with iTunes. That does not help with the next hurdle that is activation lock. Most thieves likely care nothing about your data anyway.
The thief now needs a naive or gullible customer and a quick exit to make a sale. They sell a phone that can never be used.
That's the solution. Eliminate the customer.