Ruling 2091
Six conditions must be fulfilled by the seller and the buyer [for the transaction to be valid]:
1. They must be of the age of legal responsibility (bāligh);
2. They must be sane (ʿāqil);
3. They must not be foolish with finances (safīh); i.e. they must not spend their wealth in futile ways;
4. They must have an intention to buy and sell. Therefore, if, for example, someone jokingly says, ‘I sell my property’, the transaction is void;
5. They must not be compelled by anyone [to carry out the transaction];
6. They must, respectively, be the owners of the commodity being sold and the payment made in exchange.
The rulings about these conditions will be explained below.
-Ayatullah Sistani, Practical Laws of Islam, Buying and Selling