Stocking it when it is abundant to sell it later is acceptable rather this is rational thing to do. But storing it with the intention of increasing its price in the market is a different thing. The difference between monopoly and trade is great. In monopoly you just hoard the goods to make prices go up. In trade you just buy it when it is cheap because it is then abundant and sell it later when it becomes not so abundant in the market. This is in fact a service you make for the customers.