"they have the cheapest option, among their nearby competitors, for particular goods of a particular quality."
Example: 100% grass-fed, organic ground beef at Whole Foods near me is cheaper than the identical product at a regional chain grocery store. The regional chain definitely has some dogshit beef that is considerably cheaper, but I'm comparing apples to apples.
What caused the reversal? I'm assuming Amazon's efficiencies of scale and other (oft-hated) strategies.