Every marketer and customer gets some easy benefit from a single marketplace to go with the most customers(for marketers) or marketers(for customers), maximizing the competitiveness of their respective markets. In the physical world, this naturalmonopoly is mitigated more than a little by the utility of physical proximity.
It's a bit like how social networks are successful because that's where all your friends are, but more complex since it involves multiple kinds of participants.
Amazon has filled that role online, particularly for books. And that advantage is can be leveraged for quite a premium. I'm not sure I see a nice clean solution to the problem either.