What is the ruling on a person buying up all of a certain commodity to hoard it and control its price by raising it, and is the gain resulting from that lawful or unlawful?

It is impermissible to monopolize essential goods and control their prices to force people to buy them at exorbitant rates. The perpetrator of such acts must be prevented. However, if the goods are non-essential (luxury items), or if they are readily available elsewhere without hardship, then monopolization is not forbidden. Nevertheless, a Muslim should not cause distress to people.