Whether it’s an important consumer application, a specialist app to solve a particular niche problem, or even a time-wasting game you can play on your phone, you can create a massively successful business if you build software that helps people. (Look at the rise of Slack—the team communication software that went from side project to billion-dollar company in just 2 years.)
Clearly, there's a lot of demand on Amazon, and if any product is going to sell, it's going to sell well on Amazon. But the goal here is to source the right products that will easily sell at the world's largest online retailer. Generally, products between $10 and $50 sell very well here. Just be sure to do the right market research before jumping on this bandwagon.
Considering that you have a finite amount of time, passive income should make up a large part of your work. If you're serious about generating any semblance of income online, then passive income should be one of your sole goals and ambitions. Why? Wouldn't you prefer to do the work one time and get paid repeatedly as opposed to relying on your time to generate that income? Invest the time at the front-end so that you can reap the benefits on the back-end. This means putting in a bit of sweat equity and not getting paid today. Rather, you'll get paid somewhere down the road. And you'll continue getting paid whether you keep building that passive income stream or you stop.