The "greater fool theory" is the name given to what happens when speculators get into a market. The value of the item no longer matters, what matters is that the item is "hot" - it is the "in" thing. Someone will pay $100 for something worth $75 because they think that next week someone else will pay $110, and the guy paying $110 thinks there is another guy the week after that will pay $120, and I pay $130 because I think there is a guy that will $140 next week.
$100 Fool < $110 Fool < $120 Fool < $130 Fool < Me < $140 Fool
The next fool in line is the "greater fool". It all works until there is no greater fool, then it all comes crashing down. At the end of the day, it was only worth $75.
Beanie Babies, Pet Rocks, Pogs, Fidget Spinners, Pokemon Cards, etc, etc - the collector markets are littered with the carcasses of people thinking there is always a greater fool out there somewhere...
"The Luggage had a straightforward way of dealing with things between it and its intended destination: it ignored them." -Terry Pratchett