One of the most common is the +1/+1 counter, but counter markers are also used for planeswalker loyalty, to charge abilities, and for specific card qualities
And its all because we played this weird commander that no-one seems to like
Today, you would need a small library to house every card ever printed
Be sure to practice your coin flipping before you play any of these, because resolving flips quickly is key to keeping your pods sanity
Thankfully, there are still plenty of new Mega Evolutions from Pokemon Legends: Z-A that have yet to receive their own cards, including both versions of Mega Raichu, and the newly revealed Mega Garchomp Z
None of these cards are particularly valuable at this time, perhaps due to them being very new still and the Pokmon GO Charizard, despite being both very collectable and very playable can still be bought for less than 30