What's involved in building a site like this? I am subscribed to an industry forum in the beverage space where people buy and sell equipment using forum posts - t's really inefficient and I often think they could use a simple marketplace site.
Flippa is one example. There used to be a forum where users used to buy/sell websites. Forum admins saw an opportunity and built flippa (it was called something else before that, can't remember what it was) and now raking in the dough.