Does the owner of a phone store selling a part whose price he has not yet paid count as selling what he does not own, and is there any sin upon me in using the phone with my knowledge of that?

The worker's sale of a painting left to him by its owner for him to sell is a valid sale. It is permissible for a person to sell what he does not own if he sells it on behalf of its owner with his permission. Therefore, there is no harm in your use of the device, and you are not liable for anything.