They force sellers to use their payment processor. If you don't, they charge an extra 0.5% markup on top, which basically means that no 3rd party payment processor can be competitive on price.
I think you can also measure their power in the same way as Amazon - The % of fees that they capture as a percent of sales volume goes up every year. Slow squeeze.
But then by that logic any action by any platform is never illegal. It's their platform, they can do whatever they want. If you don't like, don't use their platform, right? Is that what you're saying? I'm not totally unsympathetic to that POV, but in practice I guess it doesn't seem to be working out so well...
I think you can also measure their power in the same way as Amazon - The % of fees that they capture as a percent of sales volume goes up every year. Slow squeeze.