It shouldn't happen like this in the first place. Before building something, you should already have access to your target audience. There are many ways of solving this. E.g. doing consulting and building a list of clients with a similar problem. Starting out to build a solution without this makes it just so much more likely that you build something that doesn't solve a real problem or there's no buying interest. Ideally, you can build for a group of clients who are ready to pay when you give them an MVP.