I think the weirdest thing I've ever found at a secondhand store was a late-1800s primary school textbook about physiology and temperance. Like, here's how your body works and why intoxicants will Mess It Up.
Sometimes I still regret not buying it, but my already cluttered home probably appreciated my willpower.
Thrift stores and the like really open your eyes to the sheer amount of weird, niche stuff there is in the world.