The house for sale is a slab on grade. I’m concerned about the duct below grade flooding. The owners don’t know about a drain line or access to drain. Should there be something on a residential duct?
Water should not pentrate the embedded duct work of a concrete slab on grade foundation. There will be no means to drain the system if It should have freestanding water within the ducts and normally will have to evaporate or be manually pumped out of the system. Water should not be entering the area below the slab. I observe the duct work with a light and mirror as much as is visible from the registers. If past or present indication of water is visible in the ducts I recommend a sewer camera be used to determine the full condition of the ducts as this is the only means to see the big picture.
The topography surrounding the foundation plays a very important aspect as to water entry below the slab that I won’t get into here. I could talk for days on this subject alone. I have an infamous saying in my area pertaining to local builders. They are famous for digging a hole and building a home in it and expecting the water to run away from the foundation. Or perhaps they just don’t care.