Sale by coercion
If a person is coerced into contracting a sale, the contract is considered invalid. The Hanafi and Maliki scholars give room for the coerced party ‘s belated consent once the force is removed. The Shafe’i and the Hanbali scholars consider the sale to be invalid. According to the Hanafi jurists, if a person is forced to sell his property without will, this will be considered an illegal act — meaning that not only is the contract not valid but the coercing party is punishable.
Sale by an insane person
By agreement among all scholars, a sale to an insane person is not valid. By analogy, a sale made to a drunken or a drugged person, or any person unable to reason at the time the sale is made, is also invalid. Interdiction due to illness making the person non compos mentis will fall under insanity as well.
Sale to a blind person
The Shafe’i scholars do not consider a sale to a blind person to be valid. The majority of the jurists, however, agree that if sufficient description of given to the blind person, the sale will be valid.
Sale by a child
A sale by a child (or to a child) is not valid if the child is not discerning or capable of reasoning. There is consensus of the scholars on this. However, when the child is discerning or capable or reasoning, most scholars allow the sale with the requirement of the guardian’s permission. The Shafe’is do not allow such a sale.
Sale of the which is not owned
A sale by a person who does not own the object of sale, and does not have the permission to sell from the owner, is not valid. This stems from the Prophet’s injunction, “Do not sell something which you do not own.” The Malikis and the Hanafis allow this sale if the owner gives a belated permission, after the fact.
Source: Kharofa, 77-79.