Check the discs under a bright light at an angle before you even hand over cash, because scratches that run in a circle with the spin are usually the killer while ones going across the disc can sometimes still boot. Storage unit sellers almost never test anything, they just see a stack of games and slap a price on it. A cheap trick is to hold the disc up to your phone flashlight, and if light comes through the label side, it's done no matter how much you buff it.

Guess the chickens got a fancy sourdough starter instead of dinner.

Did you try drying the chuck with a rag between rain jobs instead of coating it?