One mistake in this category is so common and so avoidable that it belongs at the top.
Posting the record inside the jacket
The disc works against the seam for the whole journey and splits it. Take the record out — in its inner sleeve — and pack it alongside the jacket. Packing records.
Photographing the disc flat
Vinyl shows nothing straight on. Tilt it so the light runs across the surface; that is where scuffs and hairlines live, and buyers will ask for it anyway.
Giving one grade
Grades are written disc/sleeve. A single number leaves the buyer guessing which half it describes.
Guessing the pressing confidently
The label and the run-out identify it. If you are not sure, show both and say you are not sure — the room often knows, and the bidding tells you.
Not saying whether you played it
"Played start to finish last week, no skips" is the most valuable sentence in a record listing, and it costs one play.
Describing a warp as "slight" without showing it
Hold the record edge-on and turn it on camera. A warp that stops it playing is not a cosmetic note.
Posting in a flat parcel with no board
Records need rigidity, not padding. A proper record mailer exists for this.
The one that costs most
Heat. A parcel in the sun comes out bent, and there is no fixing it. Avoid posting into the worst of the afternoon, and consider collection for anything scarce.
Questions people ask
Does cleaning help?
A proper clean usually does. Say that you did it and how.
Is a split seam fatal?
No — it lowers the sleeve grade. Say which seam and how far.
Are common records worth listing?
In lots, and they build a room.