Block Dragon is simple to summon in Rock-type decks because it only requires players to banish three Rock-type monsters from the GY, which is something that will naturally occur during a combo
As new sets are released, older ones may be rotated out of circulation, making those earlier sets even more valuable and difficult to track down
And the lifegain makes it hard to race
However, as strong as this guy is, when he was in his GY, there were several times when the opponent used Monster Reborn to steal Cien and lose
New cards are Armed Dragon LV10, Magical Mallet, Inferno Reckless Summon, Ring of Defense and The Grave of Enkindling
Although not all cards are valued at quite that much, it still shows that there is a player-based market involving hundreds of millions (according to reddit, almost 3 billion) dollars worth of cards