Not Delta, but I heard when United tickets are purchased through Expedia the seats selected are not assured.
When we were flying back from Maui a few weeks ago somehow it seemed everyone's seats on the flight from Maui to Denver got switched. A woman in front of me in line was trying to get her party seated together. The gate agent told her that booking through Expedia did not assure them of the seats they selected. Interesting. I've always booked directly with the airline.
And a question, if you book through Expedia and have to make a change, or cancel, what happens? Do you get a credit with the airline for the cancellation?