If you had an online marketplace for market-research freelancers, what kind of companies and project would you target?

Maybe hire a freelance market researcher to find out? ;)

Generally, something like this is going to appeal to fortune 500 companies. Smaller companies and entrepreneurs are going to be doing market research themselves.

From the question it is difficult to tell if you have come into possession of such a platform, or are thinking about building one. Regardless, to succeed it's crucial that you deeply understand who your target customer is and what pain you are solving for them. Most ideas for a business stem from the entrepreneur's discovery of a solution to a problem for a certain group of people.

Since who has the greatest need is unclear to you at the moment, the best thing to do might be to contact people at companies who you think have the greatest need. Find a way to get in touch with someone at the company involved in hiring market researchers, and ask them what challenges they face. You can also talk with a few market researchers themselves to get an idea of where they see the greatest demand of companies looking to hire them. Hope this helps.

Fair disclosure: I am angel investor in a company called Survata which allows companies to access market research via a self-service web application.

I don't think a marketplace of market research freelancers is really filling a significant need or pain-point. Companies care about getting access to data, and generally have someone on staff or sub-out professional assistance if required.

I recognize this doesn't answer your question directly but as it seems you are trying to determine the viability of the concept, I am answering it in that context.

Hi and thanks for inviting me to answer this question. Unfortunately, when I sold vWorker.com, I also signed a noncompete agreement that prevents me from assisting companies that would compete with Freelancer.com. But I wish you the best of luck with your idea and company.