Set, printing, and finish
Identify the set, the specific printing (Alpha vs Beta vs Unlimited, for example), and any special finish or border treatment. Early-printing and foil versions of the same card can differ enormously.
Grade for the high end
For graded Magic — especially vintage and the Power Nine — the grade and grader are central to value, with thin high-grade populations driving steep premiums.
Anchor on realized sales
Value from recent sales of the exact card and grade. Slabline aggregates Magic comps and shows value by grade with sales volume, then tracks your collection’s total and protection score over time.