I have seen a cpl "everything not requiring a FFL" stores, but they usually don't make it. There is one such at Hughsville (aka Amish) Market but is only open weekend and occasional wed.
The other theoretical aproach to the ammo business would be to specialize rare and unusual cals , and primarily do business nationwide thru 'net and advertising in speciality publications , rather than walkin trade.
I think his business model was buy it at Wallmart and sell it for gun show asking prices. He must have realized the Wallmart three box limit would have limited his stock.