The system would need to have some way of checking that the parking brake \ handbrake has been applied and if necessary altering the customer while they get their ticket.
As far as I can see the main bottleneck would be the time taken by returning shoppers to re-pack their vehicle with groceries/kids/etc and drive off the turntable.
Whilst in storage, could a serious coolant or oil leak from one car drip onto the car below?