Level 1 cards need to be upgraded as do level 2 cards. When the cards are in level 3 or above they can be minted. Only minted cards can be traded or sold.
For people who already have cards in level 1 and 2 upgrading is straightforward. If they upgrade to the highest level they can then the main cost for them would be the 1HBD they have to pay as fee to mint the cards.
Thanks! Had not read this about Season 1 yet, so that's helpful