I'm going to be flying from Harrisburg, Pennsylvania to Dallas, Texas... I don't have current identification for Pennsylvania but I have an expired identification card to prove who I am. Will that work or should the person purchasing the ticket just print the boarding pass and mail it to me?
You will not collect a physical ticket from a ticket desk; air tickets are now electronic. You can print a boarding pass using any valid identification at the airport, or you can log on to your airline's website with your five/six digit reference and your surname and print your boarding pass at home. You will need valid identification to take your domestic flight, please see https://www.tsa.gov/travel/security-screening/identification