There are two factors that sell an (auto)biography: fame and relevance.
(Auto)biographies by famous persons sell because everyone wants to know what their lives were (or are) like. We, the common people, want to know how rich, famous, or talented people live. The (auto)biography of a celebrity will sell, even if that person has a completely common and boring life. Because even that is an insight that tickles our curiosity. How can they live a normal life, if their circumstances are so exceptional?
(Auto)biographies by unknown persons sell because the "protagonist" led an exceptional life or because his life exemplifies the average life. Anne Frank was a very normal, completely unknown girl. Yet her autobiography has become a bestseller because her life was both tragic and an example by which we can understand the lives of many similarly normal people at that time. Today, of course, Anne Frank is "famous", if you want, but only through her autobiography. It is her autobiography that made her famous! That Anne Frank never intended to publish her diaries adds to the veracity of her writings and makes reading them even more heartbreaking.
I disagree with the advice of Dale. If you are a common person and your life was average, you cannot make yourself famous and then sell your autobiography off that fame. And if your life was interesting enough to make you famous through the strategies outlined by Dale, it will sell without that effort, too.
If you go to a bookstore and look at the biographies section, that will be misleading because the books there are often limited to famous people. But in the novels or social/political sections, there are many (auto)biographies of people that somehow exemplify the circumstances we live in. There are many (auto)biographies by or about women who escaped oppression in islamic states, for example. These are common, average women who were completely unknown before they published their books, but they have become both an inspiration to women in similar circumstances and have helped outsiders understand the situation in their countries.
So if you are unknown, but you can manage to make your life relevant to a wider public, either because your life was exceptional, or because your life exemplifies something that people are struggling to understand, then your book will sell.
There is one type of autobiography that seems to profit from novellisation, and that is the "normal" life of a member of some fringe group or subculture. A good example is Fight Club, which fictionalizes the experiences of the author. Other examples are books about drug addicts and criminals, but also musicians or teenagers. The authors of these books are people whose lives are not normal or average from the perspective of the wider audience, but are normal and average within the community they are a part of.