What about my post is incorrect?
Guy has 2 cards
Guy tells buyers he has two cards - very clearly twice.
He sells each card individually using the one listing - ie 2 available.
When the OP buys the item there is only 1 of these still available.
Therefore he pays and gets only 1 card, exactly as he should and would have expected had he read and understood the listing.
The "postage" fee is known to the buyer before he bids so must always be reckoned into the overall cost of the item and dont forget this charge not only includes actual postage, but packaging and any handling charge that someone deems as necessary. Yes there are issues about fee avoidance and if you think that is happening don't bid a report the listing to ebay.
IMHO the OP feels a bit daft at his mistake and is trying to deflect this onto the seller to make him feel better about himself.