Make it have to follow the same regulations as other currencies and banks. If it looks like currency, and functions like currency, maybe it should be considered currency.
Usually microtransactions have such a low real price that it wouldn't make sense to make the bank transaction for just a single skin or whatever. I think you'd just end up with bundles instead where there's a single thing you actually want with a bunch of bullshit attached to justify a higher asking price.
Ya "don't like it don't play it" till every game had premium currency, which is what basically happens right now. Premium currency is a cancer on games and needs to be regulated out of existence. It's a predatory system to make you think "oh I'm not spending money I'm spending gems!" and leads to addicts losing tens of thousands of dollars. Anyone who defends these systems is either a fanboy or clueless about their impact.