I'm not a legislator, of course, but in my mind it would run something like:
- if you have over x third-party selling participants (say, 50 or 500)
- and the marketplace provides services to the "general public" (e.g. the Apple Store, qualifies, an intranet market would not)
- and the marketplace has revenue over x per day (say, $10,000)
- then participation in the marketplace must be governed by a clear, unambiguous set of rules that gives no advantage to products made/sold by the owner/administrator of the marketplace, and all interpretation of the rules, or challenges to such interpretation, must be made by a neutral, third-party arbitrator without influence from or by the owner/administrator of the marketplace
- and additionally, rules of the marketplace must not be written in order to give any kind of explicit or "implicit" advantage to the owner/administrator of the marketplate (Apple can't always feature its own Maps application over Google's, for example)
These specific rules are totally off the top of my head, but it's just a kind of idea -- once any market reaches a certain size, it's in the general public interest to ensure a level playing field and due process -- I mean, that's what things like court-enforceable contracts, food regulation, stock exchange and public company regulation, etc. are all about.