I have very little corrosion on the engine and my bike is 7 years old, 45k and beaten through 3 winters, she does get a wash and coat of gt85 from time to time.
As Jacko said, exhaust is stainless steel, it's not high grade stainless, so will tarnish, but can be polished nicely.
If it looks that pants after one winter, I'd bet it's been left outside, uncovered and unprotected.
I'd walk away- you never know what lies beneath.