Stuart 
I haven't played Monopoly for many years now. The last game was with three housemates at university. One housemate was winning and really enjoying his triumph over the rest of us. So I suggested to my other two housemates that we form a cooperative. We would let each other off rents on our properties. The standards rents would apply to anyone not in the coop. And you could leave the coop if you wanted. None of this is forbidden in the rules of Monopoly because it has little market regulation.
Within a remarkably short space of time the monopolist had lost his dominance and the rest of us had flourished. He went bankrupt and the rest of us agreed to end the game as joint victors. I felt as though we perhaps played it more to the spirit of the original game that Monopoly had been derived from and found a non-monopolist solution.
A few years ago I shared this story on Twitter and then had angry libertarian men complain that I'd cheated and had actually made a cartel, not a cooperative. They weren't happy that I pointed out that no rules had been broken. In fact it was the lack of market regulation in Monopoly that allowed us to do what we did. Plus we did it fully openly and the fourth player was able to join our coop if he'd wanted.
I still don't agree that we were a cartel as there was no secrecy, deceit, or defrauding going on. We were very open about forgiving rent and doing better deals within the coop than the 'official' prices outside the coop. Anyway it was funny to see libertarian men be unhappy about something happening due to very limited market regulation (no rules saying you couldn't discount rent or undercut the guide prices).
Nemo@slowe
From the economic analysis point of view, you formed a cartel. The difference is not the secrecy, it's that you made it impossible to compete. Now cartels are good for their members (as you learnt). They're just terrible for customers (as your competitor saw).
(Cartels are organised in secret because they're illegal...)
@iinavpov Ours wasn't organised in secret. It was all open, inclusive and open to join/leave. And outside of the Cooperative Society there was no external difference in what other players experienced. They got charged the "standard" game rents. Nobody was defrauded, cheated etc. There are differences.
@iinavpov But I can see the need to present it as equivalent to a cartel. And people could also form secretive cartels in that edition of Monopoly if they so desired. There wasn't anything stopping that.