Any tips/ recipes for a slow-cooker?

I LOVE cooking comfort food, but I’ve never used a slow-cooker before. Basically all I know is that you add a bunch of meat and liquid and simmer it overnight. So if you’ve used one,

-is it reasonable to buy a slow-cooker secondhand? How much does that cost usually?
-what kind of recipes/ techniques have you had the most success with?
-anything else I should know?

Thanks!