I've been writing sales copy and teaching sales for 15 years and I've seen both developers and laymen spend 1000s of hours and 1000s of dollars on startups without knowing who they are selling their product to. They build something, spending years in development, only to ask "how can we sell this?" after everything is built.

This is a huge mistake.

My advise is to study sales and marketing prior to building out a project you intend to monetize. Alternatively, find a partner who can sell or hire a consultant before writing a single line of production code.