[tensor]

I don't see how you can think this. Amazon is not the only payment processor, it's not the only inventory management company, not the only delivery company, not the only online store company.

There are any number of permutations of different service providers you could use to sell online. In fact, a very large number of people who sell online don't use Amazon at all. Or they will sell on Amazon as well as in other channels, or even direct.

[JohnFen]

For mainstream retail in the US, you have Amazon, Walmart, and... who?

[dymk]

Target, Nordstrom, etc?

[JohnFen]

Yes, I think you could argue Target. But Nordstrom? I don't think they have that sort of power.

Remember, an oligopoly (or even monopoly) doesn't necessarily mean there aren't other players, it means that the oligopoly is the only thing that can really influence the market. It's about who is writing "the rules", so to speak.