I find it interesting that these threads (an this one specifically) generally degenerate into how to keep ones own arse out of a legal crack (as the advice giver) like the one in the floor (that may or may not be there) instead of concentrating on helping the audience and questioner in general. This is not a new question to the RE world and I'm sure many that are reading this have run into it or will at some point.
If I were in the postition the OP is I most likely wouldn't worry about what I
have to do considering current market conditions, I would probably hire an inspector who knows what he's doing and get it inspected and cleared. Even if you lose this buyer it will most likely come up again.
The other thought I had was to do what my car dealer does with warranty work.
Suggest to the buyers a compromise, you'll have it inspected and
if the inspector finds something actionable you'll take care of the bill, if not they do. This will let you know two things, first how serious they take the crack and second, are they willing to put their money where their inspectors mouth is.
Just 2 cents from someone who's not worried about ending up in a "crack".