People low-ball me in business all the time and I don’t take any of it personally - I just smile and continue to negotiate the deal.
I think I just worked out what bothers me so much about this case: I didn’t mention in the original post, but I’d bought three expensive watches from this dealer before this. I think, over the years, I’d felt we’d developed a friendship which is why the obvious low-ball stung so much. I think I’d made it personal and hence the visceral reaction. As Tony says right below, collecting is an emotional obsession, so I think that explains my response.
NickO