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jeff"buy the card not the grade" is a mantra by which to buy cards. It is especially true the older the card is. I agree with Tom that any card with writing on it should be marked way down--seems like any card with writing on the front is almost automatically poor. But I would much rather have a "poor" card with a pencil mark on the front border than a vg card that has been graded such because of a light or slightly misregistered photo.
And that is where the real philosophical issues come in (if we want to be so grandiose about it) -- what kind of card merits such-and-such-percent of NM book value.
Many active eBay dealers seem to have recognized the impossibility of grading -- I am among the many who may offer a grade, but rely mostly on the bidder to examine the scan.
Even with slabbed cards, especially on the extreme high and low ends, prices reflect a wide variety of preferences, and a wide range of grades within each numerical designation.
All of this is just to say that it is the particular defects of a card, combined with personal preference that determines the value of the card--there will never be a grading scale that can explain the monetary value of a card without supplementary description. And that isn't a bad thing--it just suggests that attempts to create a definitive, end-of-story grading system are probably not going to succeed.