You may think the lines are fine, because you have flushed all the toilets and run all the faucets, but once you move in and you are running the dishwasher, and someone is taking a shower, or doing a load of laundry, and you flush the toilet, and all of a sudden everything is backing up and overflowing. Never buy a home or commercial building without first getting the sewer line scoped. Even brand new homes or buildings can have problems. It could be the line was unknowingly damaged during construction, or not even finished, or it may not be installed on the correct slope for water to drain properly (we have seen all these problems before).
Make sure your plumber shows you the video and make sure he explains what is needed or not needed. Also, you should be able to watch the live feed as the camera is going through your line. If the plumber doesn’t want you watching over his shoulder for something like this, then he is the WRONG plumber. Be pro-active and make sure you get a camera down that sewer line BEFORE you buy the house.
If your plumber recommends sewer line replacement, this is a good time to get a second opinion. Unfortunately, some contractors just want to make a buck and will recommend sewer line replacement when it is not necessary. We have been the plumber to give a second opinion, and often times there was very little or nothing wrong with the line.