So, who were they trading with? Fate? Titans? Even leaving the "break the rules" part aside, that is certainly another ability of Carnymancy — making a trade with higher powers.
A trade can sometimes be just replacing one thing with another, or swapping two things. There doesn't need to be a higher power. For example, the way Luckamancers give good luck to some units by creating bad luck for other units could be considered a trade. I don't think we have any clues about what sort of trades Carnymancers make.
Maybe Carnies are playing shell game with fate. For example Jojo may just have displaced Sylvia's DIAF under a different shell, so it was still in the game waiting to be revealed later under an other circumstance (shell).