If an animal was listed on MM and an inquiry was made there, then regardless of how one finalized the sale a seller review can be posted for that animal. If the sale went bad in any way, and it violated any off MM's rules (including selling unhealthy animals), and the seller didn't make it right, I would suspect that MM will help get the seller to make it right.
Yes, they've been good about keeping the place pretty clean, and I've noticed a few bad sellers have been removed in the last year or so.