In short, the builder of an experimental amateur built aircraft can apply for a builder's certificate upon issuance of the airworthiness certificate.  The builder's certificate allows the builder to perform all maintenance and inspection functions for that one aircraft including the condition inspection which must be completed at least once per year.  An experimental amateur built aircraft acquired from someone else must be maintained and inspected by the original builder (if the original builder obtained the builder's certificate and wants to do the work) or an A&P, except for preventative maintenance.  An IA is not required.