Another problem in online marketplaces is the feedback system. Not everything is ebay when it comes to these. And, I don't really know if implementing an ebay style feedback system really makes sense for people other than ebay. Like, anywhere you expect to attract sellers.
The whole point of a feedback system is to spot problems, and be useful as a means of tracking the reliability of a seller at a glance. You don't necessarily need caustic reviews to get there. Also, I think when it comes to smaller platforms, that kind of thing drives sellers away. It probably does the same thing on the larger systems, but they're so big that nobody cares.
So, my solution here is to take a page from Uber and my friends at Simbi. If you don't know about Simbi, you should check it out. It's novel. I've been using it for a year now, and it really is the best of all worlds in terms of community building online. It's not only best of breed, but the best, and potentially the only site like it that I've ever seen.
Simbi does a couple of things exceptionally well. They do the gamification in a very practical way. It should be a case study if you ever want to know how this kind of thing is done. And, beyond that, they do feedback and messaging beautifully.