What is an example of quackery?

Here's a few...: Selling apricot pits, industrial-strength bleach, the stuff in your water softener, venus flytrap juice, apricot seeds, magic water, rocket fuel, or plants known to be poisonous to profit off desperate people. The folks at the top are crooks; those in the middle may be sincere; the "science" is totally made up but this breaks no law and is protected by free speech in the usa. Be discerning.

Here's a biggie: If you want to learn about medical quackery, I highly recommend the book "Charlatan: America's Most Dangerous Huckster, the Man Who Pursued Him, and The Age of Flimflam" by Pope Brock. The charlatan in question was "Dr." John Brinkley, but there are lots of other examples in the book.