I'm not asking you to negotiate my grievance.
On the contrary, when you introduce your experience as relevant to a conversation then people's assessment of your experience is something they evaluate. That's why you share your experience; because you want others to take it into consideration. You'd have to be very simple-minded to think they'll just agree with you blindly without weighing your words.
Cast iron isn't unbreakable, and if you think it is that's very consistent with a person who packed it poorly, resulting in damage. Everything you say is just reinforcing that the system was working, you were a poor seller and rage-quit when people demanded you do better.
What, am I supposed to have them send me their broken pieces to me as a "return"?
Yes. And... "duh?" Are you an idiot? (Don't answer that)
On most purchases it says, "Buyer pays return shipping." There is no financial gain by smashing something and then paying shipping to return it. They end up out their shipping, they lost money. So yes, you demand they return it and use their refusal to return it to win any dispute. And you put a comment on their feedback that they refused to return it. (And don't add additional insults that make you look bad, just give the relevant facts concisely)
It's not rocket science. There are lots of sellers who have huge numbers of sales, and they don't have the levels of bad buyers that the neckbeards on slashdot claim. Less than 1 in 1000. People who claim most of their buyers are trying to scam them are toxic sellers. In the case of the slashdot neckbeards it's probably unintentional; they're probably just too anti-social to comprehend what the normal "common sense" market expectations are for a seller. I mean, just click on their comment history.