Exactly how invasive/detailed do you three believe most pre-purchase inspections are? And what is the legal liability that the prebuy A&P holds here? This is why there is common advice out there to perform an annual inspection in lieu of a prebuy.
When I do them, I do them as non-invasively as I can, so if I can't get a look at things with my borescope (and admittedly, I can get a look at a LOT of things with my borescope), it's not part of my prebuy without specific request or probable cause/doubt. Also I am running two different checklists (ABS + Beech), none of which have me pulling spar covers. That's in the Annual Inspection checklist, which is like 40-50 hours to run in full. Anyone signing up for a $3-4,000 prebuy?
Sellers are usually rather reluctant to have me rip their airplane apart on behalf of some rando.