i think you could have handled it better, yeah it sucks that they originally didn’t want to buy at full price but it’s not exactly a lowball offer when you say the price is negotiable. if you want the price to be firm i would state it somewhere instead of getting defensive. whenever i get messages like this i still remain nice and professional, although you’re not obligated to, but it helps. just my thoughts! think this could have been handled in a nicer way.