They don't "take." They offer a price for selling on their site, and sellers choose to pay it or not. They built that better mousetrap.
In fact, I think we should retire the term monopsony exactly because its actual usage is a subset of the usage of monopoly and nobody ever cares about the distinction.
Monopoly is the most generic term so I don't think it's 'wrong', and technically neither are correct so...
Generally speaking I prefer to buy a product directly, or from a specialized retailer.
Last week I purchased a stethoscope from an online stethoscope retailer. A benefit of doing this is that they offer laser engraving-- something that Amazon, with its warehouses of robots and robot-like employees, cannot easily do.
The specialty retailer also carries a wider array of color and finish combinations, because that's their product niche.
Using a specialist retailer has allowed me to flex on my fellow volunteer EMTs with a personalized Technicolor stethoscope, which is dumb but nice.
For examples, do the 7 bigges supermarkets in britain, Tesco, Lidl, etc, have a monopoly?
I can buy all my food from the snaller shops and market markets, as a buyer.
But if I am a seller, and I need to sell mass market food, and all the big supermarkets refuse to carry it, you will go bankrupt.
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.
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.
Only Alibaba has more market share than it. But it is indeed quite far from a monopoly.