Back in the 1970's, there was a brief fad where people were buying "Pet Rocks." They came in a cardboard box that looked like the kind of thing you would carry a hamster home in from a pet store. They had an instruction booklet with tips for "training" the pet rock.
While I was aware of the fad--I was a teenager--I did not take part in it. They sold for $4 and that was actually a decent amount of money. Also, I had very skeptical parents who would have pointed out that it was just a rock and I could pick up a rock anywhere for free. I knew better than to ask!
The Wikipedia article on Pet Rocks, cited below, says that the fad lasted about six months. They were really hot and well-known very suddenly and they dropped out of consciousness for the most part equally suddenly.
Truly, this fad was unsustainable. After all, people began to recognize, as my parents did, that they were buying essentially nothing. The hot Christmas gift in 1975 began to gather dust in 1976.
In actuality, I have seen lots of fads come and go, including Beanie Babies, Cabbage Patch Dolls, Elmo, and other things I can't even remember. These fads tend to play on people's emotions, which leads to them fighting each other at McDonald's for a Happy Meal prize, in the case of Beanie Babies. Eventually, logic starts to kick in and the "investments" people make in their collections of whatever become mostly worthless.