It's not amazingly complicated. There are - and have been for decades- IATA rules on reticketing. Expedia didn't do it properly. Delta isn't responsible for travel without the passenger having a valid ticket. Reread posts 3 and 8 about Expedia's role and responsibilities. People who don't like the answer keep trying to spread responsibility. Everyone who flies a lot has a story of reticketing difficulty with an airline but the carrier's airport employees are then available as a resource to fix it.