That would depend on the mower and it's age. How has the mower been used? Residential or commercially. MY preference would be to buy from a commercial operator rather than a residential owner. Reasoning behind that is the greatest percentage of the commercial guys will take care of there equipment because it's there bread and butter so to speak, where the residential people tend to ignore there equipment and IMO most residential owners will be buying equipment from HD or Lowes and we know what that stuff is.
If you can find a decent commercial mower with three or four hundred hours in the price range your looking in I'd snap it up in a heart beat. Even as high as eight or nine hundred hours, it's still a better quality more than a cheap residential mower. jmo.