One tactical improvement would be that when the robber turned his back to exit the store, instead of grabbing him in a choke hold, the victim should have clobbered him at the base of his skull with fist, or better grab a bottle and hit him there or alongside the side/temple. Lesson, when you attack an armed assailant, attack with the most force possible as quickly as possible. What an amazing opportunity to have the perp turn his back so nonchalantly!
Along with that, some training in aggressive defense/atack skills would have helped.
I am wondering what the law is on this situation. If they guy has been robed at gun point, is there any issue about using lethal force when he turns around to leave? If the victim had a knife or gun would he be within legal limits to shoot him since the perp could always turnaround at the door and take a literal parting shot at the victim?
"The ultimate authority ... resides in the people alone. ... The advantage of being armed, which the Americans possess over the people of almost every other nation ... forms a barrier against the enterprises of ambition."
- James Madison