Way back in 2006, I had an idea for a t-shirt for an engineer friend. I used my graphic design skills to create the art, bought some iron-on transfer paper, and made the shirt.
It looked awful.
I began researching online, and discovered Print On Demand. I had the shirt printed up, and it looked great. [He still wears it occasionally.]
Since Zazzle let me set up a shop and sell my art to the public, I put the Mad Engineer shirt up for sale ... 3 weeks later, I sold one!
I've been selling my art ever since.