My grandfather had an idea about how to improving HVAC systems (he was in the business). He discussed it with his assistant - someone he considered a friend and protege. The kid went and patented the idea (my grandfather was opposed to patents and thought all knowledge and ideas should be free), started his own company, and became my grandfather's chief competitor in the area. My grandfather died a short time later (a few days after my birth, so I've only heard about this second-hand from my grandmother, who served as the bookkeeper, my mother, and my father, who served as the subsequent assistant).

My father worked for a small company that approached a large company (large enough that I can guarantee any American has at least heard of said company) about an idea. NDA's were signed before discussing any details. The big company kept saying that they were trying to get enough details to determine whether to buy out my dad's company or simply pay a flat fee for the idea. For over a year they pumped my dad's company for details, all while working the idea covertly. My dad's company continued to pursue the idea and actually did take their version to market. The way my dad's company found out that big company had stolen the idea was when employees started seeing big company's version of the product on store shelves. 30 years later and I still see the big company's product on a regular basis, my dad's company went out of business (spent too much on development and then were beat to the national market by big company).

In both cases, if the idea hadn't been shared, it couldn't have been acted on in bad faith and the actual idea-generators would have had a chance to market or complete their implementations.

Nowadays investors are demanding more visible traction before investing in the web space, but that didn't always apply, and still doesn't apply to spaces like semiconductors, biotech, etc.

Note - There currently is no marketplace for ideas that I know of.