Our Walmart - and I understood it's a national rule - allows only three boxes per customer per day. You can play the "shop in the morning, shop in the evening" bit to catch a different attendant. That gets you six boxes a day, as long as the stock holds up (unlikely the evening visit will garner much).
Unless the customer knows the person behind the counter personally. Then, the attendant calls the customer to announce new shipment. Customer comes in, buys multiple boxes, sells on gunbroker, shares profits with the attendant.