LOL, they use the blockchain, so it's similar to buying bitcoin or something, just instead of a virtual coin, it's a virtual card. The reason they sold out, is that each card is limited to a certain number. You can trade or buy/sell these via the online marketplace.
Personally, I like blockchain currencies that trade in processing power and sell that power on the free market to be used for productive things. Mined coins like Bitcoin just seem to be wasting the world's electricity and processing power in my opinion.
As for digital cards, I prefer physical autographs and sketches. I might have bought a pack or two of these for the novelty if they hadn't sold out. I do believe the people that bought these will make a nice profit on them though.