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

Question Details: I apologize in advance but this is a little long. I purchased a home and closed on it almost 4 months ago. I did not get a home inspection because I was led to believe that the VA inspector was far more picky than a typical home inspector. This house was listed/advertised and sold as having been converted from septic to city sewer. The first time I tan my washing machine the water backed up and out of the drain pipe and onto my floor. The seller/contractor that purchased this house to flip it came and attempted to fix it 3 different times an finally called Roto Rooter and they got the pipe flowing. I thought all was good. Due to me working night shift I typically sleep during the day so I never did the laundry. Well, fast forward to early this month and my wife left to visit family. I was up doing laundry during the day and heard gurgling coming from the sink, so I went to investigate. While outside, I saw suds coming out of the vent pipe on the roof. Seeing this, I dug up the entire pipe coming out of the house that drains the washing machine and kitchen sink. The pipe exits the house about 16' below the surface of the ground and rises to only 3 inches below the surface, makes its way all the way around the porch and makes a steep drop down to what I assumed was the main sewer line coming off the house where the bathrooms are. I called the seller/contractor over to the house and he said that they didn't mess with the kitchen/washing machine line and only installed and ran the sewer line from the bathrooms to the city sewer. He said that the line coming from the washer and kitchen sink must still be going to the septic tank. So, I now have a house that is seemingly half on city sewer and half on septic with a pipe coming from the house that does not drain due to a 13' rise in elevation and a septic tank that was never drained, crushed and filled in in accordance with state plumbing code. I realize that I should have gotten a home inspection but there is no way this would have been discovered without digging up the entire pipe. Is the seller/contractor that converted from septic to sewer responsible for finishing the job properly and to code or am I on the hook for thousands of dollars in repairs?

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