Everything that breaks was ok 10 seconds before it broke, that 10 seconds could be 3 years after they have purchased the car or 3 minutes !!! We can't tell the future and deal with things of a mechanical nature, at some point they will break.
Amazingly most workshops aren't full of cars which people have just purchased so it a rarity, even more so if you prep the cars properly.
It would be interesting to know what percentage of p/x's show faults within 7 days of being traded in compared to cars customers have just purchased !!!