I've been looking into this where I live and the basic things to check are:
DBA or Doing Business As license in your state, varies from free to slightly expensive but is needed to accept money in a name other than your legal name
Sales taxes for your state, including the research of online tax requirements for your state
Health regulations for making cosmetics (which you should be following to the T, they aren't difficult)
Filing taxes as a business, including using home space and allowable deductions
While this seems like a lot of research, it's pretty simple to get these answers from your state sales regulations website it detail.
Good luck! Isn't it fun to create all sorts of fun colors? I got together with my aunts, mom and cousin to create some interesting shades to share, we had loads of fun.
Have you found a mineral base that works for your shadows? Mine took about a year of tweaking before I created one that works for my oily, hooded eyelids. In that year, I also learned how to do a mineral foundation for my super-dry skin as well as oily skin. I'm still perfecting the normal and oily skin formulas, since I rely on other people to test them for me since I don't fit those categories. If you want to discuss any DIY MMU stuff, I'm open to sharing my experiences with different products and suppliers.