I have just tried chat and the agent was the most cooperative so far. He/She saw what their website was showing and even tried to create a new reservation, but wouldn't let them save the seats with a "seats not available" error message. I also asked him/her about "long sell" but they were not familiar with the term.

Too bad I don't have enough Delta miles to try to book and cancel within 24 hours.

Does anyone have any other ideas?