Caution on deregistration

Sold a car to these folks in 2019 (Brackenfell branch). By accident saw the vehicle yesterday near Goodwood N1 City (02/12/2021) carrying the same license disk and license plates as what I sold it to them back in 2019. How is this possible that a new owner can take a vehicle without completing ownership transfer and new registration. As I recall now, the visiting salesperson did not process any official (Yellow form) ownership transfer papers when collecting any of my vehicles. STUPID ME trusting these lot. So highly recommend doing an ownership de-registration at the Traffic dept when selling to them as soon as the vehicle leaves your premises after a sale!!!