The agent didn't sign it. Buying and selling houses can be done directly between buyers and sellers, that is to say, they can also be bought and sold directly without the participation of intermediaries. Therefore, the intermediary did not sign, and the house sales contract was valid.

Article 465 of the Civil Code. Contracts established according to law are protected by law.

A legally established contract is legally binding only on the parties, except as otherwise provided by law.