if there is one thing that i have learned about grading marbles on ebay it is "forget about the grading!". no matter how hard you may try, your grading will not be the same as mine, his, her's, or thier's.
i believe the best way to go is to descirbe any damage, flaws, as-mades, etc, and try to show them clearly in your pictures. this way you can let the buyer decide on the condition/grade, "if" they take the time to read your description and look at your pics.
thanks.