Making up your own price doesn't take into account the realities of compromise between buyer and seller. Seller can set whatever price they want, but it won't sell unless some buyer is willing to pay that price. Any market system should serve to facilitate bringing buyers and sellers together to exchange goods. An actual auction can serve this purpose.
And yes, agree 100% items don't remain rare. When items become common enough to become a commodity, they'd move to the markets, since people know how to price. Then the undercutting comes in, bringing down the price to some sort of equilibrium.