Upload & Sell: On
Wahoowa -- Sorry for your travail and grief over the lens.
I'm sure the seller was caught flat-footed and had never noticed the effect -- it's easy to overlook, as your one un-fogged images shows. That doesn't mean the condition doesn't exist, I'll bet the seller didn't know, especially based on his shocked reaction and poor response. Personally, I would have done different if I were the seller.
A couple years ago, I bought a 15-85 IS on eBay, and when I got it, it had similar splatter marks (but fewer and smaller), and were very evident when I fogged with my breath. I used Singh Ray fluid to clean, but a few were permanently part of the coating. I contacted the seller, who gave me the option to return for refund or keep it and get $100 refund. I took the partial refund (knowing, like you, that the marks would need disclosing if I ever sell the lens and affect price). The good news is the marks never affected picture quality in any way, and that is one of my most contrasty and sharp lenses! I often use a lens hood outdoors to hedge my bets, but I now take it anywhere with no worries about damage or splashes!
Clean it, then take a few pictures in typical contrasty scenes, and then contact the seller again, and reference this thread. Ask for a partial refund or return. Let the person know you aren't blaming them, but the marks were there on arrival.
Good luck for a smooth conclusion.