I built a betting platform for a niche esports game and had moderate success with it. Mostly word of mouth, but it was a small dying tight-knit community desperate to attract knew players so it wasn't hard to get everyone interested.

EDIT: Working on a product for a community I was part of was definitely what helped me succeed. I worked on the problem I was familiar with, and I received a lot feedback and support from other people. It also helped me stay motivated. :)