I think that payment methods on the markets should be negotiated by the buyer and seller directly.
Payment providers vary a lot between countries, continents, and even languages. Having the market railroad everyone into using the payment methods they prefer is wrong. I mean, how exactly is that a free market?
Uh, you are already free to arrange whatever you want with whomever is willing. The fact that a site doesn't accept every form of payment, or even any form is not wrong.
The free market means that you are free to NOT associate with anyone you don't want to, not that site X has to facilitate your dealings no matter what. If you think there is some demand for a site that allows any type of payment then go for it. I can't even imagine what that would look like though.