This may be a bit off topic for actually making, but if a person is to do a business with mailing, I think mailing prices are a big obstacle. This seems to be especially the case for competing with mailing prices from China to the USA. It seems they can make and ship stuff to the USA for cheaper than someone in the USA can ship. I would like to hear your opinions on the matter. Which country you are in will help put your feedback in context. I am sure different countries have different shipping situations, after all.
I have considered the idea of 3D printing some molds or making molds some other way and making some items for sale, but the whole shipping thing seems to make the chances of a successful business a bit slim. I have also considered selling 3D printed or CNC milled goods. So far, my best chances seem to be doing local business and bypassing the whole idea of shipping costs.