A lot of these items are not being bought and sold at a massive value for foolish reasons but as a type of money laundering scheme. The super-rich move so much money around that they can't easily do it with mere cash, and trading in things like fine artwork, high-grade watches, and the like are a way of moving beyond the impossible amounts of wealth around without moving any
actual money that would alert tax collectors or other people who watch the money, and allowing actual money to be laundered in the process.
Something similar happened in the Diablo II game where high-level people had so much money they didn't want to trade gold anymore, and the magic item Stone of Jordan was traded by players as currency instead, allowing them to move around more wealth than the original system was designed for by bypassing the currency system.
To highlight this it can be useful to look at the rare and famous diamonds. The Deepdene is an important chunk of history, and hasn't been seen in about 20 years. The Akbhar Shah's been missing since around 1988. The Deal Sweetener hasn't been seen in public in about sixty years. Actually if you go through the
list of famous diamonds, a clear pattern emerges. All the big diamonds are either publically displayed in a museum/crown jewel set, or go missing, usually for decades at a time only to suddenly reappear at auction, then vanish again for decades more, being quietly traded for millions out of sight of the plebians or stored, a repository of wealth immune to inflation. Most of these purchases are just listed as "Anonymous buyer" and "Anonymous seller" so there's no way to figure out who has it now or how many times it's been traded since.
The world of fine art is also heavily associated with money laundering and schemes to move large amounts of money around. One of the reasons artists axiomatically become famous and valuable only after their deaths is because after they're dead, the art is entirely in the hands of the elite and the art can be safely increased in value at that point with all the artificially generated value going into the pockets of the elite who own it. It can then be traded around letting them swap millions of dollars without a cent being visible to the authorities. When Mexico passed laws putting
limits on how much fine art could be traded and requiring that information on the buyers and sellers be recorded for taxation and law enforcement purposes, sales of fine art dropped by 70% in a single year.
The reason for using these items is that they're immune to inflation and currency manipulation by authorities, keeping their value for centuries easily and dramatically inflating in value if a few rich people agree and sell them at raised prices, increasing the price of all similar items so that the collectors can gain large amounts of value without spending on it. This allows them to be stored without losing value, and at inflated prices they can be donated to museums, generating a tax break well in excess of what was originally paid for them. This is beyond the value they provide in laundering cash and being tradable while avoiding taxes on enormous amounts of wealth swapped around in a briefcase.
