That's fair, but Amazon has only a small % of total retail sales (4% in 2017, according to a quick search). I'm not sure at what point it's commonly assumed that a company has monopoly or market power, but I'm pretty confident it's not 4% or even close to that.
I'm not sure that's a super meaningful distinction for market power. It's a different method of selling, but ultimately you're selling largely the same things. You probably wouldn't separate out "Ford has this much market share in dealerships vs online sales" most of the time, for example.
Well, I suppose that's the key question. But I think that yuo might argue it is, because there's a huge "long tail" of goods that you can only really hope to sell online.
edit: another quick googling says that US courts apparently almost never conclude a company has market power if they have less than 50% market share: https://www.repository.law.indiana.edu/cgi/viewcontent.cgi?r...
Amazon is still growing at a good clip but I'd be really surprised if they were able to pass even 20%, let alone 50%.