First of all, if there is no damage to the pan other than stains, I'd buy it. It's a fantastic pan.
What is the exterior material, SS or aluminum? Have you tried to clean it? What with? What is the stain from?
I agree that Barkeepers Friend is your best bet for a first try. That is a mild abrasive and stain remover. It won't scratch your pan. If that doesn't work, more aggressive methods can be used.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan