What is the legal ruling on selling by word of mouth only between two individuals without a contract or down payment?

A sale is concluded by offer and acceptance, and it is not a condition for its validity that it be documented in writing or by witnessing. Delaying payment of the price does not harm it, except in usurious transactions (ribawi goods), where immediate, hand-to-hand possession (qabd) and equality (tamathul) are required if they are of the same kind. If the sale is concluded and the two parties separate without a conditional option (khiyar shart), neither has the right to revoke it except through annulment (iqalah). It is forbidden to sell over another's sale or buy over another's purchase due to the enmity and hatred it causes. Scholars differ on the validity of such a sale despite its sinfulness; the majority of them hold it to be valid while being prohibited.