So uhm, I went back to the store and told them the story and said that I probably won't buy used media ever again. They compensated me with free sticks, new in the box this time, and I went back home, my stomach still churning after the thoughts of these photos.
I sometimes think back to that woman. How the case went, is she okay today - is she even alive?
I just wanted to share, and please don't buy used USB sticks (they might even contain malware!)