I built an app plugin for Shopify called Custom Fields https://customfields.bonify.io

I must say, if you want to build something that instantly has a market consider building plugins for existing 3rd party systems that allow you to charge a fee. It ends up being a much smaller project than trying to build your own, huge web app, and it comes with a targeted, focused audience who can easily install your app with a couple clicks (and get charged). You don't really have to market anything.

Build something good and people will find it.