I think some terms are being used interchangeably that probably shouldn't be, IMHO.
Overbooked today does not mean oversold tomorrow. Overbooking occurs all the time. An oversell situation will not be known until people start showing up at the airport on the day of departure, no?
I've always thought that, generally speaking, the definitions are:
Overbook: More seats have been sold than there are seats (takes into account "no show" factor)
Oversell: More people actually "checked in" than can be be accomodated with the available seats