The way to have a buyer before founding is to have worked at an established, profitable firm that has that exact problem, cannot build a solution themselves, and get a signed agreement from them to buy your future product. If you leave that firm and have that document, you can raise money, then use that money to hire a team, then build the product and fulfill the contract.
Getting into such a position requires an established career. Therefore safe, reliable entrepreneurs cannot possibly be college drop outs - they need to be established careerists with good connections who have entrepreneurial ambitions.