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.)
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You'll also need ecommerce software, fulfillment software, worry about warehousing, customer service, refunds and so on. But that's not all. You'll also need traffic. Think search engine optimization, Facebook ads, and other social media campaigns. Sound like a lot of work? Sure, it is. Especially if you do it all on your own. You could opt for Amazon's platform, which might be the easier route. But, then again, at the end of the day, this is a serious business, which could produce significant profits. So you're either all in or you're not.