Fb already has chats attached to the listings and asks whether you sold on or off marketplace. A change to "which person did you sell to" would be trivial at this point. Prevent posting phone number or address in the listing and you'll be required to go through the chat.
Regardless of what you think of the idea itself, the notification part would by very easy to implement.
Regardless of what you think of the idea itself, the notification part would by very easy to implement.