Starting by building is a great way to waste a huge amount of time on solving a problem no one has. Start by talking to potential customers and growing an email list, not building. Aside from being less risky, it's way more fun to work on something that you know people want and are waiting for.

It also conditions you for the fact that you'll need to spend at least 50% of your time on squishy marketing stuff to get anywhere. You can't just code your way to getting customers.