Depends where you are in your process.
Taking a lean approach to prove demand will always help your case. Without having built anything significant in terms of the market functionality, you can easily have a sign up page, and drive traffic and have signups ready when you open your market - both in terms of buyers and sellers. Using this time in parallel while something is built to prove your go to market message and demonstrate traction will prevent delays later.
There are of course the pre-requisites, such as ensuring your vision is big enough to be attractive to your target investors (VCs have different needs on an exit -read multiples- to traditional business angels for example) but also starting to engage those investors now, not for money but to build a relationship, provide a high level roadmap and starting to deliver on that roadmap, will prove to those investors you deliver on what you promise. All ease the road later, removing fears so they can ultimately sign a check.