Some people don't understand the definition of negligence. To meet legal requirements for negligence the store employee had to be doing something that he knew was dangerous, or should have known would be dangerous and did not take precautions. The fact that he was trying to unload the gun is evidence that he was taking precautions to prevent an accident.
Does not say how the accident happened, but there are enough facts present to probably rule out negligence. Had the person handling the gun been under the influence of drugs or alcohol, had he been dry firing the gun without checking the chamber, had he given the gun to a 9 year old kid to unload, had he knowingly pointed what he thought was an unloaded gun at someone it would be negligence. To have a gun fire unintentionally while trying to make it safe is simply an accident.
There is almost never a criminal charge in an accident, but that does not mean the person who causes one is not responsible in a civil trial, and there probably will be one here.
Too many on the internet gun forums throw the negligence word around much too freely.