To export your book, first, you must choose the output format (in the lower-left part from any of the sections). On the right side, select the posts or pages to publish (or you can click on the "content" checkbox to choose all items listed). Finally, press the "Publish Ebook" button to export in the format you previously selected. The generation will start immediately.
By default, no posts or pages are selected, so you must choose the items to be included in your book (using the checkbox) and then click on the "publish Ebook" button. Otherwise, the plugin will not publish the book because no items have been selected
And then you are done! You already have the book in the format you want, and you just have to follow the steps of the provider to which you want to upload your book to sell.