Read an old guide that said staple rust is a killer for silver age books. They called it "rust migration" where it can spread to the pages. @kevinross is right, spotting those tiny spine ticks needs good light. Makes you realize how much you miss just looking at the cover.
At a comic book fair in Chicago, I watched a dealer inspect a Spider-Man issue. He pointed out how small spine ticks and staple rust can really change the grade and value. I started focusing on those details too, and it made judging condition much easier.