Simple, he's figured out you're the type who will pay the overinflated price for a sick animal because you feel sorry for it. Your only recourse is to not reward the seller for poor behavior and husbandry by walking away. Unfortunately it stinks for the animal.
Just tell him you're not interested, and give him the contact info for a local reptile rescue in case he can't rehome his overpriced sick animals before he has to move.