Who flies planes on ferry flights?
Ferry flights encompass much more than delivery and retirement flights.  Any time an airplane has a problem that can't be fixed on-site they can generally obtain a ferry permit to get it to an airport that the maintenance can be completed at.  

At the airline I worked for, all flights, including initial delivery, were conducted by pilots on our seniority list and other airlines I'm familiar with have the same requirements in their contract language.  Our airplanes were manufactured in Brazil and a crew would be take an airline flight down to the Embraer factory, accept the airplane and fly it to a US customs station and then on to our main maintenance airport.  These flights were fairly uncommon and often went to very senior and management pilots.  

Normal ferry flights for aircraft repositioning or maintenance needs were generally done by low-seniority reserve crews because those tended to be non-scheduled activity and show up with short notice.  

In all cases though, at my airline, ferry flights were conducted by company pilots.  This can vary by airline and there probably isn't a universal definitive procedure common to every airline.