They could do it like so many other companies.
Either minimum bid set by seller, or minimum bid sets to the deposit cost to post the auction.
That way if the card sells, the deposit is paid back out of the winnings. (No selling the card at a loss.)
The only reason to have it the way they currently do, is to ensure those who earn their gold, instead of buying it, can never afford to compete with whales.
That’s right. They give up balance in an effort to ensure real money is spent on their game in ever increasing amounts.