While I was sceptical at first, I realised it comes with interesting advantages. Instagram for instance is a great platform to post good looking pictures promoting a product, and the ability to just click on the picture you like to see what you can buy from it is a pretty interesting aspect in my opinion.
On Facebook, I would say it's a way to make people interact more so it does make a bit of sense, you get to develop connections as you could by joining a Facebook group, by interacting on a page's post or under the picture a friend of your friend's posted of them. The logic behind it can be chaotic, but it makes sense.
After saying this, I don't see just any social medium offering a place to buy and sell products. On Twitter? What would be the point? But on LinkedIn? Maybe it could be a way for companies to showcase their work, but at the same time I don't picture LinkedIn becoming a place where you can buy anything either.
Is there one particular social network you're thinking about where the marketplace shouldn't be there?