I see the benefits, but I also see the risks. A lot of the social stuff right now is up to the altruism and expertise of instance admins, which cannot always be guaranteed. Not to mention the risks that one user's abuse (eg. scams, contraband sales) could shut down the entire venture.
I think there needs to be a federated protocol to handle payments that also has some protections for buyers and sellers alike, but without the explicit backing of banks it simply cannot exist within the world of fiat currencies.
Obviously, you could build this with cryptocurrencies, but that limits your users and of course many people will never touch crypto with a ten foot pole