A contract of sale is a legal contract. It is a contract for the exchange of goods, services or property that are the subject of exchange from seller (or vendor) to buyer (or purchaser) for an agreed upon value in money (or money equivalent) paid or the promise to pay same. It is a specific type of legal contract.
An obvious ancient practice of exchange, in many common-law jurisdictions it is now governed by statutory law. See commercial law.
Contracts for sale involving goods are governed by Article 2 of the Uniform Commercial Code in most United States and Canadian jurisdictions, however in Quebec such contracts are governed by the Civil Code of Quebec as a nominate contract in the book on the law of obligations. In Muslim countries it is governed by Islamic Law ( sharia )