Bait-and-switch, plain and simple.
This is as classic as bait-and-switch scams get. You "book" your flight on their web site, and then you get an email saying that the booking is "pending". Within a few business hours, you'll get a call telling you that tickets at those prices have all been booked and offer you tickets at a higher price which they ASSURE you is still a great deal, despite better prices being available on Kayak, Expedia, and any other reputable booking site.