The Seller Pro theme demo displays an Author Box after the post content and before comments on single posts.
To add an author box to your posts, go to Users > Your Profile, scroll down to the About the User section, and add your content to the Biographical Info field. See below for the text and HTML used in the demo.
To add your profile picture (gravatar), you must have an account at http://gravatar.com with an image uploaded before it will show on your profile screen or in your Author Box.
Next, scroll down to the Author Archive Settings section of the same screen and check the Enable Author Box on this User’s Posts box.