In this category the packaging is frequently the item, and most mistakes come from forgetting that.
Posting the collectible box as the parcel
A courier label on a collector's box is value gone before the parcel moves. It goes inside a shipping carton with filling all round. Packing boxed collectibles.
Showing only the front of the box
The back and the bottom carry the shelf wear, the crushing and the price sticker. Buyers here are buying the box as much as the piece.
Not declaring reproduction parts
A replaced accessory or a modern stand is ordinary and sensible. Undeclared, it makes a collector doubt everything else in your listing.
Opening a sealed item "to check"
Sealed is a state with its own value, and opening it spends that value permanently.
Claiming "rare"
It is not a condition and rarely survives checking. Say the edition size if you know it, and nothing if you do not.
Peeling a price sticker the night before
Removing one badly leaves a worse mark than the sticker. Show it and let the buyer decide.
Not working the joints on camera
For anything articulated, loose joints are a real fault and they do not photograph.
The one that costs most
Sunlight. Plastic yellows and does not come back. Where a piece has been kept is part of what it is worth, and it is worth saying.
Questions people ask
Is a crushed box still sellable?
Yes, described as crushed. Many buyers want the piece.
Should I sell loose pieces?
In counted lots, with anything notable kept out.
Do instructions matter?
For complete pieces, yes. Say whether they are there.
More: selling toys, what they are worth.