How to test whether your idea has a market
blog.codeable.io
blog.codeable.io
If you can communicate very clearly what the product is going to look like and what it's going to do, selling a product that isn't done yet is not much harder than selling one that is. In some ways it's easier, because the customers can fill in the details themselves, and they don't appreciate how hard it is to get all those detail right.
Perhaps I'm jading at seeing too many bad Show HN posts where I think, who would want that?
I have gone in with a demo and come away with a deal for real money. Then we built the real product around that customer's actual needs, which made it dramatically better than we could have by just guessing in advance how they would use it.
Edit: Also see just about everything on Kickstarter, where people often pay real money for things that don't exist yet.