The book trade has a vocabulary and a habit of evidence. Most mistakes here are skipping both.
"First edition" with nothing shown
The copyright page settles it. Hold it still for ten seconds on camera and the claim stops being a claim.
Using "good" to mean good
In this trade good means worn. It is the most common honest mistake there is, and it produces disappointed buyers who were reading the word correctly. The ladder.
Not grading the jacket separately
A first in a torn jacket and a first in a clean one are far apart. Write it as book/jacket.
Not mentioning a price-clipped jacket
Four words. Leaving them out is the kind of omission that makes a buyer re-read everything else you wrote.
Posting hardbacks in padded envelopes
The spine takes the impact. Boards, bag, box. Packing books.
Cleaning paper
There is almost no safe way to do it. Dust it and leave it alone.
For films: not stating the region
A disc a buyer's player will not read is a return nobody is at fault for, except the listing.
The one that costs most
Posting without a moisture barrier in the monsoon. A plastic bag costs nothing and prevents the one failure that cannot be argued about.
Questions people ask
Is an ex-library copy worth selling?
Yes, as a reading copy, with ex-library in the title.
Does foxing need declaring?
Always. It is common and it is visible.
Should I sell books individually?
Scarce ones alone, the rest in subject lots.