I think that's easy to guess. His ticket was simple, just the outbound and return flights, no stops. Whatever aroused their suspicions happened after he took his first flight. Once at the destination, he wasn't staying put in some 5* hotel, but a student taking trains and buses to other cities. They would have had no way of reaching him even if they wanted to. He just came for his return flight and was surprised by the questioning.

Most likely, the seller had gotten caught in another real or sting transaction in the meantime.