Sardines are the currency used in the Black Market.
Sardines, besides Black Market purposes, are able to be converted to Bonus Points.
They can be obtained through any of the following methods:
Booster Packs come with various quantities of Sardines. Pulling a 25 or 50 sardine card will result in a cooldown being set before a similar card can be drawn again.
Sardines can be bought with real money.
Every so often, an option will appear in the bottom right corner of the shop screen that lets the player watch a video to earn sardines.
After watching a video, a cooldown is set before another video can be watched (and for less sardines!) If possible, the player should wait until the maximum cooldown is reached and a video can be watched for 10 sardines, rather than a number of Sardines and Bonus Points.
Sardines are also known as Rare Points in the Debug Menu.