> Tragedy of the commons is a problem cartels face,

That's my point. If RealPage were creating a cartel, they would be creating themselves problems without any upside. They aren't receiving the rent. They're receiving money from landlords who are happy with their pricing model. The fact the RealPage does not seem to experience the same problems common to cartels suggests that they aren't facilitating a cartel.

Signing up for the service gives the members immediate access to the price the cartel is setting. Defecters have to wait for the market to react to RealPages updates, so there is a disadvantage to defecting. Even if there wasn't, the biggest landlords would want to join the cartel because they know that fixing prices makes them richer, even if some of the value the cartel creates isn't captured.