Posted By:
MWDarin,
The 1984 OPC YZERMAN is probably authentic. As Elliot has indicated, KSA has graded enough of these so that they probably know the difference between an authentic example and a reprinted one.
As for the 1951 Sawchuk, I would be far more worried with the low feedback rating of the seller and his ability to deliver the card than I would be with the item's authenticity.