The main difference is the retail companies still vet the suppliers before letting them duke it out for shelf space.
Monster may have paid Best Buy to be on the shelf, but at least I have some guarantee that Best Buy actually bothered to check that the HDMI cable is actually an HDMI cable (and if it wasn't, would notice and pull it eventually).
Amazon lets anyone sell anything, and then takes a cut in multiple ways.
In the US, each area usually has Costco/Walmart/Target, Best Buy/Apple, Home Depot/Lowes, Staples/OfficeDepot, Kroger/Albertsons/regional grocer, Nordstrom/Macys/Kohls/Old Navy/Gap.