The current situation is like this.
I am a company in need of a consulting firm to do some project. Lets say hardware engineering.
One of two things happen: 1. I use google search and look up "hardware engineering firm". I then see 2-3 that meet my criteria and I set up calls with them. Go from there. 2. My company has a long history of using one of the big consulting firms. Let's say Deloitte. Since I don't want to rock the boat, I'm just going to set the job up with Deloitte. Deloitte is then going to source a hardware engineering firm and act as an intermediary.
It's really dumb, but that's how it is.
The primary issue I see with your idea is that a lot of projects that companies want consulting firms to work on are actually confidential. They don't want that to be public. It would have to be a mostly 1 sided marketplace, where consulting firms make profiles, and prospective clients can easily sort through them to find a firm that makes sense for their specific project.