Looked at a house we are considering buying. The owner had been doing laundry, laundry room is in basement. The whole house smelled like sewer gasses. The listing agent said it was from the P-trap not having any water in it. Is this true or should we be skeptical about buying this house?

The whole house smelled like sewer gasses. The listing agent said it was from the P-trap not having any water in it. Is this true This could be true if the floor drain trap was empty. To take care of this so it doesn't happen again fill the trap with cooking oil or anti-freeze.
However, it could be a partial blockage downstream from the washer that the discharge hits and bounces back sending a bubble of sewer gas ahead of it. Check the floor drain by putting a stick down intro the trap. It should come back with about 2 inches of moisture on it. Good luck, Tom