I could go to any teaching forum right now and find you thirty people who won't pay for Bingo Card Creator, and/or think it is a sin to charge poor little teachers $30 when they only make $60,000 a year and have to buy their own paper. This isn't maximally relevant to the business, because I can constructively prove that there exists an addressable channel to sufficient teachers who will pay that make it viable (on BCC's scale, at any rate).
This is one reason why I really like MVPs, by the way. You can often get a close approximation of an actual customer acquisition strategy running on them. Then, see if they'd take at least some action which suggests willingness to pay. ("I can't find the buy button. Where is the buy button!?" has worked for me, at least once.)