Buying a single family home or a townhouse? The Seattle Times has a great article you should read to educate yourself on sewers, A dirty secret: Side sewers can become a homeowner’s nightmare:
Across Seattle, the risk of breaks in side sewers is high primarily because of their age: About half of Seattle’s housing was built before 1961, according to the Census Bureau. Tree roots and soil conditions also pose potential threats.
The risk is borne by owners since you are responsible for your connection to the main, regardless of where your property line is.
So if you’re buying ask if a sewer scope is available, if not, you should seriously consider getting one, or splitting the cost with other interested buyers. Even with new construction single family homes and townhouses that replace the sewer line, we’ve heard of instances where workers accidentally dropped tools and debris down the brand new sewer line compromising it.