In my experience writing nonfiction books and publishing them on Amazon.com is an effective way to make money using your skills as a writer. You don't need any inventory or to buy anyone's get rich overnight kit (for only $XYZ dollars more than it is worth.)
That said, selling nonfiction books on Amazon requires doing some research to find a need or question to answer. Research your topic, including keyword analysis, and write a short book that answers a specific need or question. If you did your keyword research well it will soon be selling. Amazon offers some helpful tools to use in promoting your book and using Facebook , blogs posts and other interactive media will boost sales more.
The key to making money is to duplicate the process with other books as well. If you have 5 or 6 titles selling between 25-50 copies a day you'll see fairly decent earnings.