Based on my own experience why they use chassis number to verify the purchase of parts is because different make year might have different engine design(maybe newer tech) even though the car model looks exactly the same. The chassis number will also determine the make year.
IMO TS is being overly paranoid. The guy could have been startled by your reaction. You should have calmly ask for his business card to see if he's telling the truth.
The local AD could be the sole supplier of parts here to monopolize the market such that car owners LL go back to their workshop. They might not share information with private distributors to lose customers. So these ppl might have to gather information on their own to bring in the relevant parts. You should have asked for more info instead of turning aggressive. It may benefit you if you get to know more about that distributor.
The Chinese got one saying...if you have nothing to hide, there's nothing for you to fear.