> Name another online retailer that's bigger/better known.
You literally named two. My father knows how to do only two things on his computer - check his email, and buy stuff on eBay. There is a "Walmart.com" desk and in-store signage in 5,000+ physical stores.
You can't compare them to the entire retail market. If anything, their competition is Wal-Mart's online service, or Ebay.