CHICAGO—Cameron Percy thought he’d found a rare treasure when he moved into a new place two weeks ago. Someone had left behind dozens of Beanie Babies in a box in the basement.

The 28-year-old took to eBay , Reddit and Instagram, and concluded the small stuffed animals, a craze from the 1990s, will one day soar in value, like the bitcoins he bought in early January.

“A lot of them are never going to come back, and so they’re definitely going to become valuable,” said Mr. Percy. His online listings include Slowpoke, a sloth, and Teddy, a holiday bear.

Not. So. Fast.

Two decades after the great Beanie Baby boom, where old and young hit Hallmark Stores around the country in search of rare examples, the speculators are back, living in an alternate Beanie reality.