Forums are like a discussion platform as compared to Facebook groups which are completely unorganized. We can’t see the reputation of the user at Facebook groups, crappy search feature, repeated questions etc… The only advantage is the ease of access, a dedicated Facebook app icon on your mobile phone.
So having a dedicated discourse app of your discourse forum with notifications will add traffic to your site, in other words, its the ease of access.
I agree with member @Bill, a plugin to rate sellers/traders will be an advantage over Facebook groups. There is a thread on this already but we are still waiting for the plugin at Is there any plugin to rate sellers? - plugin - Discourse Meta