I guess you need to put together two things in this question, your capability and the needs of potential customers. Your capability is going to drive a great conversation as long as you know very well the subject. The people that have some needs that are related with your capability are going to start talking about their problems and pains and here is when the magic happens. You have great conversations because people is interested to get some expertise from outside and if you are the people that can solve it, they will be willing to provide information to solve the problem. Once you find several people with the same UNSOLVED problem, then you have a niche opportunity. Just validate that they did not found some solutions because the solution does not exist and not because they search in the wrong place.

This is a great coaching process; I had help hundreds of people to sell to new customers. Usually I was seating on their side participating as a support person for them, taking notes and watching how the conversation went. I always wanted to tape the conversation, but this was not done because requires the authorization of the customer, that do not know my role and less that my mentee was under training. I wonder if you make it as an open sales training session to disclose to the potential customer in the same way that a call center tell you about the recording of the conversation and you help your customers thru a website that create the sales feedback training process.

For me sound great a idea, that needs some work to find the best way to go.

To build an online community you need to show your expertise and leadership in the subject. You can start by blogging content in the subject and start attracting interested people in the subject. It is a rule that the number of people that reads is 3 to 5 times the number of people that collaborate in the site. You break this rule by having more and more people involved in the discussions and when you have great quality producers of content sharing ideas and answering questions you are on track to grow the community.
From the investment side, it is a very complex issue because this subject the people needs a higher level of trust of the people they are taking. I foresight a great challenge in sharing information about deals, then I will find how the share the learning of the subject as a way to gain trust.