When recently played cards have been transferred to a new owner or delegated to a new player, they cannot be used in battles for a period of 7 days since they were last played.
This is an important anti-abuse measure that prevents owners of multiple accountsfrom farming too many rewards with the same cards.
Cards in this Cooldown Period are indicated by the Timer icon being lit up on the Collection page. When hovered over, the small pop-up will say exactly how much time remains on the timer.
If Player A delegates a card to Player B 2 days after that card has been played in battle, that card will still not be playable by Player B for 5 days.
If Player A lists a card on the market just after playing it in battle, then it sells 24 hours later to Player B, the new owner (Player B) will not be able to play with that card in battle for 6 days.