There can certainly be seats showing on ITN when the flight is being oversold. By definition, overselling is the sale of more tickets than there are seats. This means you can hold a ticket for the flight that has no more physical seats available for assignment. In that case, while you can still see avaibility for a fare base, and you can purchase tickets, you will not receive a seat assignment. A 744 can be as much as 50+ oversold (I've seen this) and still not need volunteers at the end (I've been there).