He was asked—may Allah have mercy on him:

A man has secured an arrangement with the authorities whereby a particular commodity may be sold only by him. That commodity is available only from him in that locality, though it is available in nearby places, at a distance comparable to that between Misr and Cairo. Is it permissible to buy from this monopolist or not?

He replied:

Praise be to Allah.

As for the man himself, it is unlawful for him to do this for two reasons. First, he prevents others from engaging in lawful trade. Second, he forces people to buy from him on whatever terms he wishes, thereby wronging them by raising the price.

As for the goods purchased from him, if he bought them with lawful wealth of his own, buying them from him is not unlawful. The purchaser is the one being wronged: whoever buys incurs no sin, nor does what he acquires become unlawful merely because the seller has wronged him. In such a case, the prohibition applies only to the wrongdoer, not to the person wronged.

But if he bought those goods with wealth he had wrongfully taken, then that is wholly misappropriated property, and buying from him is like buying from someone who has misappropriated property. The ruling on this is clear.