Around here that would go for 150 real fast and would sell in a week for 250. At 400 it would sit for a while. Leaving grass clippings on the mower is a "trick" to show you that it was running good just when it was left there. But to me, it shows that they didn't clean it off to keep it from rusting and probably didn't give it regular maintenance.
If you want it, offer 200. But inspect the deck, under the deck (Flip it up on it's side), the blades, and the gear box (leaks from the seals, oil level and oil color). I wouldn't go over 250.
jb