But this only works for cards found in main sets and to a lesser extent special sets since they're usually kept in production for a while (and if what TPCi said in the past is true, will continue to be printed as long as there's demand for 'em and they're not rotated out), this rule doesn't work for promos since they're usually only given away during a certain window of time and given a limited print run that's long enough to fit the predicted demand
Some items can quickly become hard to find
Enthusiastic Study CMC: 2R Type: Instant Rarity: Common First ability: Target creature gets +3/+1 and gains Trample until end of turn
You can find these Buy-a-box promos at certain local game stores
Prismatic had the right idea where it banned fast mana like Mana Crypt and Sol Ring, tutors (ranging from Mystical Teachings to Demonic Tutor), and then any other degenerate cards like Memory Jar, Necropotence, Yawgmoth's Will, or Gush
Hence, you can build a deck that focuses on filling your graveyard with cards that you can later exile to create zombie tokens or use cards that allow you to repeatedly return cards from your graveyard to your hand or library to create a constant stream of tokens
