The first is that, erm, yes, there's a huge market for startup ideas. It's called the get-rich-quick-scheme industry. Sells billions of dollars worth of goods every year trying to teach people what business ideas will make them rich.

The second is that it depends what you mean by "idea." If you mean some generic one sentence pitch about some vague idea ("Uber for pets"), then, sure, it's probably not worth much.

If you have a very precise plan that includes details about go to market strategies and execution, and that is based on data only you are in possession of, it can be worth a lot of money. That's essentially what Peter Thiel talks about when he mentions good startups being based on "secrets" (the way I understood it, anyway).

That there's no liquid market for such ideas doesn't tell you much about their value.