The report came back from the inspection, and my realtor says it's most excellent; they only want me to fix one thing. Can you guess? Yes, there's a blockage in the main sewer line.
It could be as simple as getting the thing roto-rooted (couple hundred bucks?) or as complicated as getting part of the sewer line replaced (several thousand?). Gah.
Still, it's doable, and it's only one thing, and once it's done, the house is sold, sold, sold!