Always
If you are concerned about a buyer viewing your ride that may be a thief the answer is to never leave the ride unattended at any time while the buyer is viewing it, Never give out and hints or security codes about the ride until the buyer has shown you some identification and handed over at least a deposit for the ride.
The key to protecting your ride is to be careful where you’re parking it, especially at night. Make sure the area is well lit and frequently busy. Makes sure you check all your doors are locked, sunroofs and windows fully closed and any valuables are removed.