Selling bulk Pokmon cards on platforms like eBay or TCGPlayer incurs fees such as commissions and processing fees
Plus, it can be run as a Sultai commander, which is a significant advantage
The Commander Marisi, Breaker of the Coil is a 4-cost 5/4 with the ability to lock your opponents out of interaction during combat (and thats all combat phases, not just yours) and goad their entire board if one of your creatures manage to sneak damage through to them
More than anything, these moments seem to be self-perpetuating, where everyone gets tremendously excited about one particular card, which creates demand, which raises the price, which exaggerates the rarity, which creates demand The next most expensive right now is the Latias ex at almost $200, then the gold card version of the Pikachu ex at $100, and that Hydreigon pictured above at about $95
As of publication, you can still find copies in the wild for just over $5, so if you are going to push the boat out