I took a look, and there was a LOT of rust on the frame, especially at the seat stay attachment points - top and bottom. The bike rode nicely, but I decided against buying it because there was too much rust. I don't know the owner personally, and it was clear that the bike was stored outside 24/7.
Thanks for your replies! I was thinking that some rust might be acceptable, perhaps because as a kid our cars and farm equipment often had a fair amount of rust but still worked fine. Upon further reflection I can see a big difference between cosmetic rust on the body panel of a car vs a rusty bike frame that could collapse catastrophically. I'll aim for something with zero visible rust on the frame.
It's a good idea to look at the Recycle Bicycle or bike co op. So far I've avoided the co ops here because they're only open during prime family time. I'm thinking I'll spend more time there when my kids are older and can learn how to maintain their bikes. There is a Recycle Cycle nearby with more typical opening hours so I'll take a look there and at some other used bike shops.