As a buyer, its a really good idea to give a machine a good once over. Then discuss any and all concerns with the salespeople or manager of the business, to have these issues addressed before purchase. If you brought that machine into my dealership with that under carriage looking to be soaked in salt, mud, sand (all of which can have corrosive properties) I am not sure I would be able to help you either.
And most warranties only cover rust through, not surface rust. Again, why its important to inspect first. Its also important to maintain a machine by caring for the finish.
Its unfortunate to be in a situation where you bought something and find a flaw after the fact. The problem is, now you own that flaw and have to deal with it. I bet we all have had a similar issue with something we buy now and again. It is what it is.