The reservation system will sell you a seat, or not sell you a seat, as to what's it programmed to sell. But, If you can risk it, wait until 24 hours before the flight, and all of the seats sales/reservations should reconcile out. I've checked seat assignements 48 hours before a flight (every seat filled), and 23 hours before a flight for check-in (suddenly seats open up), and there was a difference... too many to just be coincidence.
The anomoly appears to be Northwest's Platinum Elite, at least from their web site. As a Platinum Elite, you can book a seat on an overbooked flight, and go to the top of the list to board, even if you don't have an assignment. The Gold/Silver/Non-Elite are the first to ask to volunteer, or be kicked off. But for that priviledge, I presume you'll be paying full Y fare... you'll get on that flight, but it's gonna cost you. The irony is, what happens if there's too many Platinum Elites? I've been on some DTW
flights with over 50% Platinum Elites... who gets kicked off first? Date/time of purchase?
If you can't book a flight, to me that says it's not just sold-out... it's overbooked. Wait until 24 hours before, and throw the dice.