What is the ruling on selling GPS tracking devices to car rental agencies, knowing that they enable them to know the locations of the cars around the clock?

There is no harm in selling GPS devices to car rental companies because there is a legitimate reason for the car rental company to know the location of its car when needed and in emergencies.

However, the lessor is not permitted to use this device to spy on the tenant, harm them, or for any other unlawful purpose.