Yeah, I've considered Walmart, but it kind of defeats the purpose for me. It doesn't do a lot of good to replace one monopolistic behemoth with a different monopolistic behemoth.
Amazon does feel pretty unavoidable for the long tail, though.
Hm, anyone here want to start a company?
I haven't researched the industry extensively, but I'm guessing that companies like to keep customer data to themselves and curate what they think is the ideal checkout experience. (Everyone should copy McMaster, though.) That's why you see companies happy to delegate the financing involved in buying their products to credit card companies, but still make their own website/checkout/fulfillment rather than letting, say, Amazon do that for them. (Shopify does seem to have quite a lot of traction, however.)