No I'm with you on this. I understand you can't guarantee anything. But a sag in a roof? Come on! Ray Charles can see that and this guys been TRAINED to see those things. That's the issue
I didn't see a sag in the roof mentioned, but I could have missed it. But, it really depends on the fact if it was obvious or not. And there is the issue of is the roof was "accessible." The roof could have been too steep to get to. For instance, in Louisiana an inspector does not have to look under a raised house (and is not liable for things that can only be seen under the house) if the area under the raised house is not "accessible". "Accessible" is defined by the ability to lay on your back and do a full rotation to your stomach without hitting anything. That requires a pretty big space. There is also the issue, as Superchuck pointed out, that the buyer was aware of the potential issue.
So, while the possible liability of an inspector is very limited, it is there. It's the reason why those guys tend to spend a lot of money for E & O policies. Because when they are liable it tends to be for a lot of money.