I think it still works this way. Pretty sure it does. That on FB (if using the app) you can leave a positive or negative mark for a person from any conversation with them in the chat window. (don't have to have bought or sold an item from the person.)
So if someone sends me a chat message offering me $10 on a $100 item I have for sale, polite me says I will reply to them and politely decline their offer. But if they get angry at my declining their offer, and if they are using the app, they can leave a "frown face" or whatever FB calls it on a "only FB sees it" part of my account.
But if I don't reply to the person at all, they are not able to leave me a negative on my hidden profile. So what's a person to do.
This hidden positive/negative rating is different from the normal 1-5 star rating you see on a person's buy/sell profile (which is only public after they have 5 ratings).
Also, it's hard (impossible now?) to disable the box on the chat window that shows the last message has been read. If you get a message and then click on the chat window to read it but don't reply, the other person sees that you read their message. So if they sent a lowball message and you read it but don't reply, they might get angry at you for not replying.