Ask HN: How to collect information to precise a vague idea of project?

Since I've heared at the T.V. that firewood was the cheapest source of energy in France, I have a project idea: "wood stove and firewood market". It can even be more precise: "a wood stove and firewood market Webapp that connects manufacturers and customers".

But I need to know if people would be interested in paying to use this site, and if lots of people would use it.

I also need to imagine highly-specialized features that would interest manufacturers that they would not find nowhere (and idem for customers), so that my Webapp would have a real added value.

What are the steps to follow to answer those questions? Could I simply go to their forums and asks them their needs?