From what I can tell, the protections put on newer phones by the vendors has massively crushed the stolen phone market. Modern phones are locked to a google or apple account and basically only usable as a) parts for repair or b) if you can phish the password from the original owner, you can unlock them.

Basically, discouraging people from buying used phones, lest they be stolen, is a way less effective mechanism for reducing theft than this kind of structural change.