I would say transparency and a formal written statement between business, versus an agreement that is so unethical or illegal that nobody wants to put their name on it or officially declare it to the public.

A business agreement to mutually not enter one another's competitive domains is a conspiracy against the consumer. Similar examples include price fixing, anti-employee "poaching" agreements.

Permissable business agreements include joint ventures, plans to supply one another when creating or entering a new marketplace, etc.

Nothing.

Collusion is functionally an illegal (and informal) business agreement.