A sale, being simply one form of contract, to be valid, and binding on the parties thereto, must have all the usual essential elements that are required in any contract. The parties to the sale must therefore be competent to enter into the contract, the consent to the terms of the contract must be genuine, the parties must not be dealing under a mistake of fact, there must be a mutual assent to the terms of the contract; the sale must be of a thing the transfer of which, under the circumstances, is lawful, and there must be the consideration, the price in money paid, or to be paid, to sustain the contract.

In addition to these requisites, and in order that fraud may be prevented, the statute may in certain cases require that there be a written memorandum of sale, to evidence the sale and to direct that unless this requisite is looked to, that the contract shall not be allowed to be good. It is true also that a sale cannot be made of a chattel, so as to transfer the title thereto where the title is in some person, other than the person making the sale, no greater title can be transferred than the person transferring himself possesses. To this rule that only the legal holder can pass a good title to the property, there are some important exceptions, the discussion of which will be had at a later time.