Grading the same way twice
Updated
You are pricing against yourself repeatedly, and an inconsistent grader prices badly in both directions.
Grading drifts
Grade a flat of poor material and the next average specimen looks good. Grade after a show full of excellent material and your own stock looks worse than it is.
Grading also drifts with mood, light, time of day and how much you paid, and none of that is visible from inside.
Write your own scale
Define your terms in words: what makes something fine, good, average. What damage moves a grade.
Write it once and keep it beside you. It does not need to match anybody else's; it needs to be the same next month.
Include the damage rules explicitly, because that is where inconsistency costs most: a chip that drops a grade one week and does not the next is a pricing error in both directions.
Reference specimens
Keep a physical example of each grade, from material you are not going to sell.
Grade against them rather than against memory, which is the same discipline this property applies to condition checking and to color matching.
It takes seconds and it removes most of the drift.
Conditions
Same light every time. A specimen under a halogen spot and the same specimen in daylight are being judged on different criteria, and lustre in particular is transformed by lighting.
Same magnification for damage inspection.
And not immediately after buying. Everything looks better on the day you acquired it, which is the single largest source of over-grading in this trade.