Who would be at fault if you backed into an illegally parked car that you could not see from your rearview in a private parking lot?

You have a right to back out of your legal parking spot. If you use your mirrors and do not see that there is anyone behind your car you are able to back out. If a car parks illegally behind you THEY are not only guilty of illegally parking but they are the cause of the accident. All it takes is an extra one minute for the illegal parker to find a parking spot that will not cause an accident.

We make choices and when we choose to park illegally any accidents are the fault of the person chosing to park behind cars that could be moving out at any time.