That's true, but the viability of other options limits their ability to set prices or exclude competition.

But it has absolutely everything to do with determination of monopoly status. If a seller can simply stop selling on amazon and sell with Walmart/EBay/whatever instead, amazon clearly has no monopoly. Same on the customer side.

Fair point. I still think Amazon is engaging in anti-competitive practices (by just selling on the same marketplace they maintain).

But my argument was poor.