The post was generic.....not directed to any individual.
As this is a somewhat UK Centric Forum....I was trying to use the UK ANO as a reference.
I can add the FAA Regulation if you wish.
The basic premise is the aircraft has to have an equipment inventory done, be weighed, and a CG determined based upon that data.....and that be the starting point for the Crew to determine the Weight and Balance of the aircraft when prepared for their particular flight.
If the data provided to the Crew is in error then naturally their calculations are based upon a false premise but due to no error of their own.
You were sharp enough to smell a Rat when your calculations kept coming up out of tolerances and then do a followup to determine what that cause was.
I would assume if the prospective buyer went through with the purchase he had the aircraft properly weighed and its correct CG determined.
That discrepancy would have raised some other Red Flags I should think.
FAA Information....
https://www.faa.gov/documentLibrary/.../AC120-27E.pdf