Doing your homework is no guarantee of success... It certainly helps, though.
Easiest way to start the marketplace is to be a large part of one side of the market first. It's possible to do it other ways (pivot showcasing working into selling work) but it's usually in a much narrower vertical.
Yeah, someone pitching a marketplace idea makes me wince, very hard to pull off from scratch without an existing following or building off of something else.