Were they brand new releases or the copies of books returned by the stores/distributor and then dumped via Costco?
Returns have the covers ripped off before being sent back by the newsstands, dont they?
We're talking BA12 here, not Iron Man 12. The number of newsstand copies were considerably less. Tearing off 1/3 of the FC of remainders was a practice by some distributors prior to direct market sales. Some distributors would clip corners or mark them, others would do nothing and just trash them or box them up and leave them in the corner of some warehouse.
Of course, direct market sales leftovers just went to the 10 cent box, as the retailer owned them and needed to get anything they could for them. Only speculating, but I imagine the Costco bricks came straight from the distributor, not returns from the newsstand. They were probably part of the (over) print runs that were neither sold nor distributed.