This wasn't the first appearance of Superman, which was Action Comics #1. However, this is the highest price at auction for any Superman comic book. It was graded as a 9/10. Apparently found in the attic where the buyer purchased it when it came out. She had told her children that she might have some valuable comic books somewhere in the house.
Their mother bought the comic, along with several others, when she was nine and living in Depression-era San Francisco. Over the years, the mother occasionally insisted to her sons that she had “rare comics somewhere.” But she could never remember where, and the boys assumed it was just a family legend. The mother died just before the pandemic, and the house sat untouched until the brothers were ready to go through it earlier this year.