On a whim, I was reading about the Beanie Babies craze (and subsequent bust) of the 90's - and this quote from the Wikipedia entry struck me:

Following are key factors that contributed to the collectible nature of Beanie Babies:

• Unique creative elements
• Supply/demand
• Availability
• New releases / retirements

Basic stuff, for sure. But it's kind of incredible that they haven't yet tried selling Beanie Baby NFT's, for ultimate irony.