I really doubt that many of those genuine repair parts for iphones and macbooks are stolen, in the sense that somebody loaded up a pallet and took it from its manufacturer without paying. Apple doesn't manufacture most of these things, particularly the ICs and screens, and relies on a whole ecosystem of vendors and subcontractors.
If a third party is paying a reasonably agreed upon market price to a factory to buy extra factory run of stock (example: DRAM ICs, or touchscreens), that's not theft.
You would think that those factories would engage Chinese law enforcement if a significant percentage of their output was literally being stolen without payment, since that sort of thing affects their bottom line and is clearly a crime in their mainland china location.
For people interested in this general topic (parent poster here mentioned living in Shenzhen), go read through all the historical content of Bunnie Huang's blog...