Show the past projects that got successes, Some important questions ask the guy to find about domain keyword and how much traffic he can bring per day.
Most of the people comes with fake proof so please becareful.
Well, we have to spend money and it's not a small project or maintained by one or two persons. Forums always consume a lot of time and depend upon active members.
You might heard about this one The Admin zone forums, which took lot of years in development.