Under certain circumstances an agent of the buyer can act on behalf of the buyer to enter into an agreement to purchase real estate that is binding upon the buyer without the buyer's signature on the contract. In some states this authority to bind a principal to a real estate contract must be in writing, in other states it may be an oral instruction to the agent by the principal. In almost all states, if not all states, the authority to enter into a contract to buy or sell by an agent for their principal must be agreed upon prior to entering into the contract. Just the act of entering into an agreement by the buyer for representation by the agent does not necessarily give the agent the authority to bind the buyer to a contract unless that agreement expressly includes that authority or the agent is subsequently given that authority.

Since real estate law, in this case what is known as the 'Statute of Frauds,' can vary greatly from state to state you would need to contact an attorney competent in real estate law in the state the property was located in to get a definitive answer that takes into account the particular circumstances surrounding this question.