I wish they would just stick with completely mystic/religious things rather than pseudoscience. Why claim that magnets or colloidal silver work when it is easy to disprove? Why sell stuff that is poisonous and actually harms people?
Pseudoscience is a cheap way of making your bullshit seem legitimate, because it draws on the fact that most people accept that science is the way we know things about the world, but many of them can't tell the difference between actual science and science-flavoured woo.
Magnets are easy to use for this because they already seem almost magic, and they have a shitload of legitimate applications. If you don't know any better, it's not hard to think "magical healing powers" is a perfectly reasonable addition to the list.
As for why people end up selling actively harmful stuff as woo, well, law of large numbers. If you sell bullshit without caring what it actually does, then there's a non-negligible probability that it will do harm*. Enough people selling bullshit, some of it will be poison. And then there's the opportunity cost, which makes things much worse.
*Conversely, there's a non-zero probability that it will help, but it's much smaller. In general, any random change in a complex system with multiple interdependent parts is far more likely to break it than to improve it.
