The exploitation of the strategy can be limited by two factors: "labor" and "capital". It sounds like labor is the more limiting factor, though.

Also, some of these prices aren't set at maximum profit per item, but there are other factors involved. For example clearing warehouses or attracting customers.

In many cases, I suppose Walmart should be coming up with a way to sell direct to Amazon, but that may not be what they want, strategically speaking.