This trade has a short vocabulary and a strong habit of grading two things at once. Keeping them apart is most of the job.
The states
Sealed — unopened, seal intact. Mint in box — opened, unplayed, in the original box. Boxed, incomplete — say exactly what is missing. Complete, loose — every piece, no box. Played with — real wear, described.
The box, separately
Shelf wear, creases, crushed corners, a torn or cracked window, price stickers, sun fading, writing. A box is graded in its own sentence or it is not graded at all.
The item itself
Paint — rubs, chips, overspray, touch-ups. Joints — tight, loose, or floppy. For articulated pieces this is a real fault. Yellowing — show it against something white. It is age and light, it is common, and it does not come out. Stickers and decals — applied, unapplied, lifting, reproduction. Smell — storage and damp travel with the item.
Reproduction and replacement
Any part that is not original gets its own line: what it is, and whether it is a reproduction, a replacement from another piece, or a modern stand. Undeclared, it taints everything else you have written.
Unused words
"Rare" is not a condition and rarely survives checking. "Vintage" is an age. Say the state, say the flaws, and let the room decide what is rare.
Questions people ask
How do I describe a repaired joint?
As repaired, saying what was done and by whom if you know.
Does an applied sticker lower the value?
Usually, for a collector. Say whether stickers are applied or still on the sheet.
What if I cannot tell whether a part is original?
Say that you cannot tell. It is a much better sentence than a confident guess.