I was ten years old and would find them laying around. The temptation to strike them was, well, tempting. I'm glad I was smart enough not to. Never accured to me to sell them- I would collect them and attached them to each other forming belts. Who knows how close I came to getting hurt!
Injury wouild probably have been superficial, a burn at most, unless a piece got in the eye: that could get infected. The shells wouldn't have been worth much then; amateur interest in 50 cal is fairly recent.