This important question should be asked, and answered, by all authors before starting to write a book. At least, if the book is meant to be sold to anyone outside of the author's immediate family. Is the book meant to be sold to a niche market, a regional or local market? Is the book meant to be a tool to create business for a larger business?
Authors who self-publish need to consider the business aspect of selling books. If there is no potential readership for the book, there will be no market for it. This means no sales of the book and the author will not only lose the investment made, but be discouraged about any future efforts at writing and publishing.
Authors who self-publish must not only determine who will be interested in reading the book, and therefore purchasing a copy, but how the book will be retailed to the potential customer base. The opportunities for sales can include Amazon, retailers who specialize in the topic of the book, non-traditional retail outlets (hunting books in a hunting store or automotive repair books in an automotive parts store) or at speaking engagements at conventions. The potential for possible sales outlets is long and not the point of this blog.
As always, authors who self-publish must do their homework. In this case, the homework is determining if there is a market or demand for the book, how the book will be distributed and where the point of sale will be.