The only truly fair way to do this would be to do it "double blind". Both graders receive scans of the same raw books. Neither knows what the books are, or what the other person grades them at. A third party would be responsible for finding the books (of various grades, naturally) scanning them and receiving the respective grades from the two competitors, the books are sealed and sent to CGC. When the books are returned to the third party, only then are the grades revealed to the two competitors and if they wish, they can compare notes. The grader with the lowest margin of error wins. If you wanted to get even more detailed, have the two graders prepare their own grader's notes with various defects and why they gave a certain book a certain score. Grader's notes can then be purchased from CGC and compared. The loser would agree to donate a small amount to a charity of the winner's choosing. Fair?