Is the seller/contractor responsible for inadequate plumbing after the sale?

They may be responsible if you can show that they were aware of the issue or problem but chose to not disclose it: a knowing or intentional nondisclosure of a significant problem or issue can be fraud, and fraud could provide a basis to sue and recover compensation (e.g. the cost to correct the situation). They key is you have to be able to show that they knew, or under the circumstances, logically must have known, that this line would cause a problem. If they reasonably did not know there would be an issue–and if they did not live in the house but were just renovating the house to sell, they may well not have known this would be a problem. If that’s the case then they would not be liable, since if they did not know of the problem, they would not be required to disclose it.