IMHO
A pre-buy inspection is just a cursory look at an airplane for the obvious things that may cost $$$ to make correct
along with the condition of the logbooks and paperwork to be sure that kind of thing is in order.
An A&P or AI can/should not be held as responsible for any thing found during a subsequent annual inspection.
If someone wants a plane to be as good as just out of annual (which may still have items missed) pay for an annual and not a pre-buy.