Lucky Duck Games
In the rules its never mentioned that sacrificing a card is a player action.
Or how many cards can be sacrificed in a turn.
As many as all activated effects says you to do so.
The player can not sacrifice at will?
No, and that is also the clue of the game. You have to manage number of your cards and its limit.
So does this mean the only way to use a card with a sacrifice effect is when you or your opponents play a card that triggers a sacrifice?
...or your card effect allows/orders you to sacrifice a card.
I.e. Shield of Zira effect has to be triggered by effect that orders or allows you to sacrifice a card (Potion of Life, Crystal Skull etc.)