Fifty Shades of Grey. The Shack. The Christmas Box. Your Erroneous Zones. Amanda Hocking's Trylle Trilogy. There are many bestselling books that began as self-published titles, and were later republished by traditional publishing houses.
Is self-publishing an on-ramp to traditional publishing?
Yes and no. Certainly there are plenty of traditionally-published authors who elect to take more control over the publishing process - and with distribution platforms like Amazon, this is made even easier. But generally these are authors who have already achieved a certain amount of recognition by taking the conventional publishing path - authors like Stephen King, J. K. Rowling, Barry Eisler, and J. A. Konrath.
More common is a mix of both. The publishing path you choose really depends on what your goals are. Stephen King, though he's self-published many works through Amazon and Barnes & Noble, continues to publish traditionally as well, at a variety of publishing houses.
The reasons for choosing one platform over another vary. Barry Eisler and J. A. Konrath actually have a book about making that decision called Be the Monkey. (Warning: Contains reference to inappropriate simian-amphibian relations.) For a writer who's just starting out in publishing, it may be necessary to self-publish simply because the barriers to entry to traditional publishing are so high.
But if a traditional publisher then approaches the self-published author with an invitation, the author needs to think carefully about the benefits and drawbacks. We covered many of these in Self-Publishing vs. Traditional Publishing.
If your self-published book does well enough, it can certainly be a first step to traditional publishing. But it doesn't have to be.
