I'm pretty much intrigued by this thought as well. Maybe the simplest way to achieve this would be by way of custom topic fields and taxonomies. I know, this is bordering on Flarum running as sort of a CMS. But let's say we have a plugin or maybe in-built functionality to add geolocation information for each topic. If not that, then just a custom field we could add displaying a drop-down with names of cities. This effectively "tags" each topic with a location and could be utilized for searching as well. Another custom field or taxonomy defined to specify if the topic is "For Buying" or "For Selling". Using the existing tag system could result in erroneous tags for topics.
So, the structure would be Categories or Channels of Forums with Generic Product/Category names and all the other information to classify would lie within the topic itself in the form of location and topic intent.
Not really sure if it's possible but just a fleeting thought since I would need to implement a similar project and I'm really keen on using Flarum.
