During my first year in university, I would put my notes on Notewagon - a service that lets you sell your notes to other students. I wasn't charging very much at all, but people seemed to like them and I was making some pocket change.
I had the idea to give my notes a proper layout design, put them on my own webiste, and sell them for what they were worth. Our school's rivals sell shirts that say “Wuck Festern”, so I called it Wuck Exams. The first version was a static site with Gumroad handling payments.
By the end of the first exam season after it's launch, I had made 20x more than I made on Notewagon. Working with a friend, we rebuilt the site as a Rails app using Stripe for payments. We're still growing every semester.