Your ideas work well in theory, but suffer from problems in practice.
When commercial sex is legalized, demand increases and girls get trafficked in to increase supply. Yes, there are advantages--sex workers can more easily get access to appropriate medical attention or get help when they are attacked by customers, for example. But you also get kids being raped. Sometimes the benefit of an activity to society is substantially outweighed by its cost and you ban it.
Consumption-based tax makes a lot of sense (we have some form of it in sales tax), although it creates tracking and complexity problems, especially if you want to have a progressive tax or are a small business owner.
And then you have criminal usury, payday loans, and people who are gambling away money they don't have--people who get desperate enough or lack information or psychological ability in a way that means they can be easily taken advantage of. As much as I like libertarianism, there are just some places where the government should step in and say "it's really not okay to make money this way."