both
If you are driving the backing car, no matter WHERE the other cas is located, YOU are the one at fault if you collide with it.
The person backing out I am asuming other person has finished reversing and is leaving if I understood correctly
take it easier on the clutch as your reversing
Car A is at fault b/c Car B has the rightaway.
When he causes the accident
The two best driving games are Parking Mania and Central Parking.
If he was driving forward past his intended parking space to take yours, then he is at fault. Cutting across is not permitted in parking lots although many people do it anyway.
The benefits of having a car reversing camera are being able to have a better overview when parking the car. This enables the driver to park the car without colliding with other cars or objects in the way and to assume an efficient parking position.
Usually the driver of the vehicle at the rear, as he/she should be paying attention to what is ahead of them. If you saw him/her reversing out you should have stopped and waited. If you did and were stationary, then the person reversing out is at fault, but you would need witnesses to prove it.
Police
A vehicle traveling in a parking lot has established use of the lane and the right of way. A vehicle in a parking space and backing into the lane must yield to oncoming traffic. Therefore, if a vehicle is backing out of a parking space and pulls into the path of an oncoming vehicle that has already established the lane, the vehicle backing would be at fault for the accident. However, if the vehicle which established use of the lane had an opportunity to avoid hitting the vehicle backing out of the space and failed to do so, there could be comparative negligence on both vehicles or on the one who had the opportunity to avoid the accident.