Anytime ANYONE calls me 'dear'...sarcastic or not (sometimes they seem to think it's appropriate but it just grates my nerves) I offer a stunning smile and respond thus:

"Did you need some help with that, dear?"
"No, I think I've got it, sunshine." with just the slightest twisting of emphasis. Those that get my meaning, smile in return...often, quite often, they laugh. Those that don't get my meaning haven't the sense to get sarcasm either, and aren't worth my effort.

So it's all good