So long as it intonates correctly at the 12th fret, it personally wouldn't concern me in the slightest if intonation at the 24th was marginally out when measured with a tuner. In fact, given all the potential variables, I'd be surprised indeed if it happened to me perfect, but maybe that's just me?!!
None of that helps you at all, but what I'm saying is, I suppose, is that do you think the buyer's expectation is reasonable? It's good of you to be concerned, but you've no idea about their level of competence.
Definitely check the string though!!