What about building a module on Dolphin that works like a storefront (with products categories and a cart) for the members?

If you build a niche community, would be nice if members could sell and trade stuff to each other (and to the casual visitor) directly from their profile.
The site owner could then get a small fee on every sale.

A proper integration with a an existing e-commerce platform could work as well.
The market is there and a module like that could be huge.