I believe that every financial transaction has the potential to be adversarial, there are many ways outside of investing that the poor are disproportionately exploited (lotteries, debt interest). I think the libertarian perspective is that we shouldn't have protections that prevent people from making personal choices that result in harm to themselves, but only from harming others. I think that transparency for the companies seeking investment, or education of the investors themselves are better ways to restrict investment. Net worth/Income is not a fair criteria.