Well also remember that to win the “buy box” is also a competition amongst people who are just drop shipping similar items, sometimes by the pallet load, directly to Amazon warehouses and then based on margin basically doing a constant reverse auction to win the box the customer sees. Amazon also knows all this and will adjust its own competing stock/listings according to these changes as well. I have also heard that their prices are similar to Walmart and will vary based on estimated income of the customer. Have an iPhone or Mac and you are probably paying more than someone that shops on a cheap Android phone or no name PC.