You're right, there's no cost up front. And if my book sells, everything will be great. Unfortunately, PA doesn't promote books at all, beyond the very basics (sending book announcements to a list of friends and relatives that you provide, sending info out to libraries, etc.) If you want your book to sell beyond just your immediate circle of friends and relatives, you have to do your own marketing. That's where things can get expensive. Brick-and-mortar bookstores won't usually stock PA's books because of quality issues (PA will tell you that brick-and-mortar stores order their books all the time, but that's a different thing from stocking them. If a patron goes to a physical store and requests a book, the store will order it for them. But you don't get many sales that way, compared to what you would get if the book was on a shelf).
My hope is that my book is good enough to get word-of mouth recommendations on top of the publicizing that I'm doing for myself, and that it can succeed. I'm not looking to be the next Oprah book of the month, but I certainly hope my book will be well received.
I put some excerpts on my website, along with some other samples of my writing style, to help people decide if they think my book is worth buying. So far the feedback I've gotten has all been very positive, so I have hope!