"Cleaning and mopping" will NOT cause water damage of any level. And if it my miracle did, the guy would absolutely know how the damage happened - not an "I guess this is what happened". Because he's flat out lying to you (and couldn't even give you a decent lie), I'd pass.
Personally, I'd tell him "I really like it and even with the slight water damage, I'd have been interested in buying it. But, because you chose to lie (horribly) about the damage, I can't trust you about anything else. How many other things don't work that you're lying to me about. Nope, sorry no deal. I'll look elsewhere."
