That's a really good question. There aren't many ways to distinguish the two cases except maybe this: on Amazon's platform, Amazon takes zero risk in hosting ot

That's a really good question. There aren't many ways to distinguish the two cases except maybe this: on Amazon's platform, Amazon takes zero risk in hosting other sellers. They don't buy inventory that they don't know will sell, and they don't pay to market products to customers, and they don't even need to hold the inventory at their own expense. They can just sit back, collect data, and enter high-reward markets while letting sellers do all the hard work of finding them. It's not quite so easy for a supermarket to do the same.