Carding games on that scale would be impossible, All Steam games get Revoked within 3 month if they are carded, along with most EADM Keys. Short term, its great, but as Online Key Store has been around a while, i seriously doubt any foul-play is involved.
Usually, Most keys come from countries where its much cheaper [Once in a while steam,EA etc Blacklist these keys], and plus, when buying in bulk with a long-time supplier, massive discounts are applied.