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Petey1. There will never be a consensus as to what is acceptable and what is not.
2. Even as to things that many would not be troubled by, e.g. erasure of light pencil marks, or soaking in water, there will rarely if ever be affirmative disclosure because SOMEONE will not want the card or will want it less and therefore such disclosure could affect the price. Does that make the nondisclosure fraud? Maybe. Depends. "Prolly" (I like that) a complex case by case question.
3. My best guess is that on an awful lot of cards, we have a lot more to worry about than erasure of light pencil marks.