Roof membranes coming unstuck from the underlying structure is not at all uncommon on RVs, and if contained to a small area it seems to rarely if ever causes significant problems. Obviously, a puncture in the membrane is a completely different matter.
I would suggest a visual inspection while traveling at highway speeds if possible (e.g. with a spotter in another vehicle or a camera mounted on the roof or something). If a large part or the entire roof is billowing up, you probably would want to pass or at least take that into consideration in the price.
