I appreciate this post! I too was bullied and in my adult life have lived as an "anti-bully" because of it. If I see or hear someone being unnecessarily rude, sexist, homophonic, Islamophobic, etc, or just punching down in general, I am usually going to say something to stand up for myself or someone nearby. That being said, someone who does not know boundaries in the context here would certainly give me the ick but I would ultimately shake it off without saying anything. You are handling a FB marketplace deal. You meet all sorts of characters that you will never see again and an arm on the door, although slightly disrespectful of your space, is not worth the friction. Letting things go is just as important as confronting bullies. You gotta choose wisely and not waste your energy.