I say monopolies and supply and demand do not exist because a monopoly usually develops from buying out other companies.
Now, with supply and demand, all items on the market have unlimited supply, so a balance between the limited availability and the demand for a certain product don't add up well.
With monopolies, the reason someone sells their buisness is because a sum of money comes in exchange for the product/company. Because of the unlimited supply of one certain item you can sell, selling off your products and store usually isn't worth it, because now another person owns rights to a product that has unlimited supply. You'd ultimately losing money if your business is decently successful.
I hope that makes sense?