A buyer mailed an offer to purchase a property at the full price and terms of a listing agreement. Which of the following essential elements is missing to form a valid contract?

Consideration refers to something of value exchanged between the parties involved in a contract. While it is a necessary element of a valid contract, the buyer's mailing of an offer implies that consideration is present, as the buyer is proposing to pay the full price in exchange for the property. Therefore, consideration is not the missing element in this scenario.

Earnest money is a deposit made by the buyer to demonstrate their seriousness about the purchase. While it is often included in real estate transactions, it is not a mandatory element for contract formation. A valid contract can still exist without the transfer of earnest money, making it not the missing element in this context.

Legal purpose refers to the requirement that the contract's objective must be lawful. Assuming that the property transaction itself is legal, this element is likely satisfied. Therefore, legal purpose is not the missing element in forming a valid contract in this scenario.