I'd further suggest that creating apps for clients is incredibly profitable. You don't necessarily need sales on apps to make money. Businesses, organizations, and people have problems that they are willing to pay to get solved with an app. Take your time selecting the right problem and you can make a good living and enjoy what you're creating at the same time.
Since I was advocating client work in my previous post, I'll go with the latter. Start with what you love. As an example I love aviation. There's a lot of money in aviation (as well as a lot of debt). Companies and FBOs sometimes have very specific needs that could be solved with an app: scheduling, customer loyalty, logging, etc. Reach out, talk to them, tell that you love what they love, just see what happens.
Also I have absolutely no marketing training and create apps in an organization that I have no control over the clientele, which actually works out great for me.