In such a world, I imagine a good number of investors will only want to trade when there's an overseer who can promise violence to right wrongs, because trading is historically a great way to get scammed. At the moment, given the state monopoly on violence, an organization like the SEC is a good way to do that. There are other ways to accomplish this without a state monopoly on violence, but the ones that come to mind don't sound like worlds I want to live in.