How do I sell an idea for a product?

You can't sell an idea because you don't own the idea. Try this link for a seven-minute web video interview on exactly that question. And this one for a four-minute link.

And I don't mean that you didn't think it up. I mean simply that the only legal ownership is an invention, for which you'd need a patent, and patents are expensive, take a long time and don't really guarantee protection.

Companies don't even want to talk to you about your idea, because for all they know, you've thought of something they're already working on, and if they talk to you about it, you'll be able to say they stole your idea.

The way real people with real ideas get value from them is by building a company to implement those ideas. Then you can own it because you own the market, you were first, you're branded, and everybody else is just imitating you.