You may be thinking, "Why should I audit my theme if it is selling already so well?" Or, "My theme is on the Genesis Marketplace and doesn't need to be audited." Or, "I purchased this from the Genesis Marketplace, so the theme has already been audited." Or, "I developed this theme for a client who won't know." Or, "I had this theme developed by another Genesis developer, and I am just not sure about it."
First, themes on the Genesis Marketplace have not necessarily been audited. Genesis Marketplace themes are developer dependent. While StudioPress recommended these themes be audited by myself or Gary Jones or another developer, the submitting developer may or may not have done this because until now, audits were expensive. Thus, the theme may or may not be written to WordPress standards and/or best practices.
Second, themes audited almost always reveal something, which will lead to easier maintenance, lower support costs, code confidence, and happier customers! Customers always find out how good or bad the code is/was that was written. The theme/site may come into conflict with a plugin that they want to use. The theme/site may have issues with the Genesis/WordPress upgrade cycle. Whatever the case, they either return to you, try to do it themselves, or hire another Genesis developer. Even themes/sites I've built six months ago give me shivers when I go back and work on them. I've worked on themes that were originally developed by many Genesis developers and surely there have been Genesis developers who have worked on sites that I originally created. If, however, I had the theme/site audited then I would have most definitely produced a better product and had more confidence in my code.