The AIO Marketguide @Maldesto posted says that negative feedback is not jsut for scams:
"Negative: Negative feedback is meant for trades to where you have not received the item you have paid for or someone has taken away money given to you for a service. This can also be left if the buyer and/or seller is extremely rude or demanding, these will be handled in a case by case scenario by the Staff Team."
https://osbot.org/forum/announcement/57-aio-market-guide/
So basically if someone is a MM and holds money and then decides to flame the guy whose money hes holding and telling him that hes going to give the money away even though the buyer hasnt recieved what he paid for that would also be a situation that would deserve a negative feedback acording to what maldesto posted.