This obviously depends highly on the domain/industry that you are in.

I can create a market place for, say, legal advice, tomorrow and have plenty of demand. I just need to make it a market place for free legal advice from top-lawyers. However, the supply will be hard to come by. Or I could get plenty of supply if I make it a marketplace for $1000/hr legal advice, no education required for the supplier. In which case the demand will be lacking. It can be quite tricky to find the equlibrium (indeed one might not exist) and I don't see how any generic advice could help. If you have managed this though, your problem should only be about reaching people. In that case, you either need to find the "home turf" of your suppliers or your buyers depending on which party you lack, go there and announce that people are looking to pay them money or people will help them solve their problem.