I've made parts for my own car since I was in high school. What is different these days is that when I design a new part for my car, I make 50 parts instead of just one, and then I put the other 49 parts up for sale. If they sell I make more.

I average about one new part a month using this method and I now have about 150 products that I sell. Every product I sell is a copy or a verson of a part that I first built for one of my own projects.