A virtual, central location where commodities are traded are commodities exchanges. The market worldwide began trading in Agro products such as Corn, Wheat early in the 18th to 19th century where Chicago due to its geographical location and being the key east-west transit point with railroad access became a hub for agricultural trading with the Chicago Board of Trade (CBOT) formed in 1848 that recently merged with Chicago Mercantile Exchange (CME) is the most successful futures exchange worldwide.
These exchanges became a legal entity to enforce rules & procedures for trading standardized commodity contracts; these markets trade trillions of dollars daily.
Many exchanges around the world trade in physical as well as in derivatives. Some of the well-known exchanges worldwide for agro goods are NYMEX, NYBOT, ICE, KC, MGEX, DCE, ZCE, TGE, MCX & more.
The primary and foremost reason by which the price of the commodities are regulated in any kind of market is the volatility of Demand & Supply, which is further governed by the number of buyers and sellers, products homogeneity, the ease of transfer of the goods and more.
One question that arises here is how much the movements in Index worldwide regulate the prices of commodities in actual markets?
There are various cases of that in history that support the above statement like the Silver crisis of 1980, The US stock market crash of 1987 & 2008, or be it the battle in the Soybean pit in 1989, throughout history there have been events where the markets have, irrespective of the main forces of price equilibrium regulated or not wrong to say manipulated the physical market.
This happens in many forms such as through rumors or false information, with rigged trades or pegging by which the prices in Index are set on an artificial level creating a false urgency or scarcity in the physical market resulting in chaos among retailers, wholesalers, and everyday consumers, one such type is the market power manipulation in the derivatives market where the trader buys large quantities of the contracts and in turn easily regulate the price of the underlying.
The common goal for all the practices remains the same to create an artificial price for the sake of profit, and it is virtually not possible to detect what caused that artificial price. There are various agro commodities under this radar, mainly Soybean, maize & Wheat.
There are various commissions formed to protect such practices to safeguard the local markets & consumers and the farmers around the world. Still, despite all the necessary measures, these practices are continuous around the world.