I have over 800 positive reviews on eBay. It’s never appropriate for anyone to receive a response as the OP did. Even low ball offers get a ‘no thanks’ response from me. If I’m very firm on the price I don’t respond. Sometimes I’ll play along with a high counter offer if I’m more flexible.
I occasionally buy things to resell and make a buck, but what’s wrong with that?
I will make offers of maybe 25% off, if that’s what I’m willing to pay, and I’m often surprised that sellers accept. I don’t want to make offers to offend a seller, but I probably have. That’s just part of selling.