Is buying and storing corn seeds to sell them at a higher price during planting season considered monopolizing, and is it true that the maximum profit margin in Islam is 30%?

The forbidden monopoly (احتکار) is to buy a commodity when there is a dire need for it and its price is high, and then to hold onto it, waiting for its price to increase further. However, there is no harm in a merchant buying a commodity when it is cheap to sell it later when its price rises. To determine the permissible profit margin, please refer to Fatwa no. 48187.