There aren't too many ways to call people out on MM (they do try to keep things 'professional', in both the good and bad senses of that word). The 'report' button on each ad isn't really supposed to be used for this sort of thing -- that's my impression anyway; you could try it and see what kind of response you get.
If a potential buyer sees a mislabeled ad and wants to get it corrected, they're supposed to send a polite message to the seller. (I've done this, and sometimes it is productive and sometimes I make yet another enemy
). After you message a seller, MM sends you a form you can fill out to note how the inquiry went. If a seller gets enough negative feedback through those forms, MM will (I assume) investigate.
They've been quite responsive to my input on other issues with ads (they don't tolerate ads for clearly illegal animals much at all), but I would guess a seller would have to get a handful of 'mislabeled' complaints to get any action taken. Part of the problem is that most mislabeled animals I see there are from high volume flippers, and so the number of complaints they'd get would be a relatively low percentage of their total sales.