I help b2b companies in finding their most profitable customers and have worked with several companies creating marketplace business models. If I was in your shoes I'd:
- Sell, sell, sell. Even if you're manually connecting buyers with sellers at first. Ensure what you're offering is a genuine need. People hate change (not a bad thing) so you have to offer them something that is leagues ahead of what they already use. Meaning it's a lot faster, cheaper, easier and funner then what they already do. Talking to potential customers is the easiest way I've found of figuring out where the pain/problem is and how you can address it.
- Ensure your market is specific, but big enough. Who are your customers and how many of them are there? At first you need to target a segment of the yacht/boating industry, own it and get people raving about you. Then you can move into bigger/adjacent categories. This proves traction and revenue and shows investors where you can grow into.
Feel free to call me if you'd like some more specific advice for your situation and stage of growth.