I am in Texas

I am not an attorney. So just my view.

Have you get a seller's disclosure at the time you signed the contract. You need to read the disclosure. If the disclosure state there is mold water damage exist, the seller is disclosured to you. You can not sue him. If the disclosure stated no mold or no statement about water damage, you need to prove the seller know the water damage (for example, you find when the seller bought the house, there is seller's disclosure stated water damage) you have a case. Or you can find the seller change the wall, He should know the water damage there. You may find who did the repair. Or check with insurance (any insurance company shoul be able to find if the house has insurance claim). Any way, you need to provide evidence that seller fraud.

another chance, has your realtor(buyer's agent gave you a book about mold.) it is the recommendation of Texas Association of Realtors that every Realtor giveing a special book to the buyer about mold. If your agent did not give you the book and did not ask you to do inspection. You may sue the real estate company. (sue the broker, not the agent. )

If you can not find evidence of seller fraud and the real estate did all his work. That is your responsibility.

For your reference.