Assess/review the technical and economic feasibility of the idea.
Assess/review the sustainability of the startup you are going to create to exploit the idea.
Assess/review the "business plan".
Assess/review the opportunities.
Assess/review the risks.
Assess/review what the competitors are doing.

You can assess/review all such aspects yourself or, even better, you can ask a third party to assess/review them for you... it can certainly help catching the opportunities and handling the risks while following your business plan...

Based on what I have been observing in a number of cases, generally speaking, it really depends on what the startup is doing or proposing to do.

When the required technologies/competences can be easily found in the market, it is highly possible to survive without CTOs and/or co-founders without specific experience/competence. When the new Technologies are being developed as part of the activity of the startup than it is highly recommended to involve people with specific experience/competence (and vision, enthusiasm, capacity to integrate new solutions, etc...), since the earliest stages of development.
In both cases it is highly necessary to invest time and attention in the careful definition of requirements and specifications for the work that (external) development teams will be asked to implement.... this means that if no CTO at least someone with a clear functional view must be present (internally).

In your specific case, I suppose that to be really original creating a "marketplace for freelance developers" you really need to involve asap very skilled and very creative developers who, at same time, have the capacity to specifiy and describe their "dreaming marketplace". whether they need to be co-founders or not is largely a matter of "relationships" and "sharing of risk"...