In the broadest category on the site, the mistakes are the broad ones.
Writing "rare"
It is a claim, it is rarely tested, and a room that collects the thing will know. Edition size is a fact — give that instead.
Not deciding what carries the value
Edition, condition, completeness or provenance. Pick the one your piece sells on and build the listing around it, rather than describing everything equally.
Using the collectible box as the parcel
If the packaging is part of the value, it goes inside a shipping carton. Packing collectibles.
Not saying how certain you are
Certainty is part of worth. "I believe this is the first issue, I cannot confirm it" prices honestly and defends itself.
Opening something sealed
Sealed is a state with its own value and opening it spends it.
Treating a certificate as the proof
It is a statement by somebody. Name them.
Packing without thinking about how this object breaks
What sticks out, what is rigid and thin, what is porous. Three questions, one minute, most damage avoided.
The one that costs most
Describing a reproduction as an original. The one line in this category that ends a sale permanently. Counterfeits policy.
Questions people ask
What if my item fits no category?
List it here and describe it thoroughly. Well-described odd things sell.
Is a childhood collection worth listing?
Often, as sorted lots. Condition and packaging decide it.
Should I research before the show?
Enough to describe it correctly. You do not need a valuation to run an honest auction.