It’s not overstock in the sense that the store had these units and couldn’t sell them to begin with. The retailer was never expecting a second batch of TMP Kirk to arrive at their warehouse but rather Spock. This inevitably caused a delay on TMP Spock shipping with that retailer, as the store contacted buyers who had received Kirk in error (the outer boxes has Spock labelled on them but contained Kirk, which the store never picked up on).
They were then subsequently recalled, as every buyer was contacted and asked to return their purchase, which was then shipped back to Exo. By the sound of it, it’s not even like Kirk was meant to go to any other retailer at the time, so Exo had a good number of these to sell on, albeit privately with whomever they made a deal with.