Did '73 Sports come with fiberglass tanks? He might not want to put modern gas in it. Can't blame him really, the additives in use these days does a number on old tanks.
Of course, if it's a steel tank, then I'm not sure... maybe he's not too excited about selling it, and doesn't want it to smell if it's going to stay in it's climate controlled room?
If I were in the market I'd be more bummed about the repaint than the non-running status.