This is a category where buyers bid on what you demonstrate, so every mistake here is a demonstration not made.
Not weighing it on camera
Weight and purity are most of the price of a plain gold or silver piece, and a seller who will not put it on a scale is asking to be bid down. Let the figure settle.
Not showing the hallmark
Show it close enough to read, and put the HUID in the listing where there is one. A buyer can check it themselves, and the fact that they can is what makes the listing believable. Selling jewellery.
Writing "genuine" or "authentic"
About a stone you have not had tested or a watch you cannot document, it is a guarantee rather than a description — and one you cannot support. Say what the piece carries and where it came from.
Hiding that a piece is unhallmarked
Old family gold is frequently unhallmarked. That is ordinary and sellable. Silence about it is not.
Polishing silver hard
Heavy polishing removes detail and, on anything old, value. A gentle clean is normal; say what you did.
Describing a stone you cannot identify
"Untested, sold as seen" is honest and sells. A confident mineral name you were told by somebody else is a claim with your name on it.
For watches: saying "working" loosely
Show the second hand moving. Say when it was last serviced, or say plainly that you do not know. Say what is original and what has been replaced.
The one that costs most
Writing what is on the outside of the parcel. Small, valuable and labelled is how a parcel goes missing. Packing jewellery.
Questions people ask
Should I get a piece valued first?
For anything significant it helps. For most pieces an honest weight and hallmark do more.
Is a missing stone a deal-breaker?
No, and it is the most commonly hidden flaw here. Show the empty setting.
Can I sell a watch that does not run?
Yes, as not running, at that price.
More: what it is worth, describing condition.