You can "sell" an idea before a product has been created. You can do this simply by talking about it, by producing a prototype, by creating a promotional video, etc. Get people who are excited to buy your product as soon as it's ready... but do this before it's ready, to prove that your idea has a market!
As Flimzy described. You basically describe the "idea" to potential buyer and once you see it's useful for it and he will tell you that he would pay for it, then you can start building it. You can even sign with him a Letter of Intent or Future contract which can help you raise external money if that's what you are after.
You can "sell" an idea before a product has been created. You can do this simply by talking about it, by producing a prototype, by creating a promotional video, etc. Get people who are excited to buy your product as soon as it's ready... but do this before it's ready, to prove that your idea has a market!
Read more about all this in The Lean Startup.
As Flimzy described. You basically describe the "idea" to potential buyer and once you see it's useful for it and he will tell you that he would pay for it, then you can start building it. You can even sign with him a Letter of Intent or Future contract which can help you raise external money if that's what you are after.