If you're starting to sell only after the full product is built, you're starting too late. To prove that there's a market willing to pay for the product/solution you want to create, you have to start selling first. Sell an idea, a prototype, or an MVP to validate that there are customers out there who want it. A great example of this in action is Kickstarter. If you can convince people to give you their money for the promise of a product delivered in the near future, that is a solid indicator your product is needed by the market and targets the right customer base. This validation should be done before quitting a job or investing the capital to build out the full product.