For a valid contract of sale to exist, three essential elements must be satisfied: (1) mutual intention to sell and buy; (2) clear identification and agreement on the subject matter (merx); and (3) agreement on the purchase price which must be serious, fixed or capable of ascertainment, and sound in current money. A valid sale agreement is concluded when the parties arrive at a valid and binding agreement, regardless of whether the purchase price has been paid or delivery effected. The purchase price may be fixed by the parties or determined by reference to an external standard. Conduct of the parties, including undisturbed possession over an extended period and payment of rates and taxes without payment of rent, can constitute evidence of a concluded sale agreement even in the absence of formal written documentation.