One mistake in this category produces a return every single time, and most sellers have never heard of it.
Not signing out before posting
A phone, tablet or laptop still linked to its previous owner's account can be unusable to the buyer — not difficult, impossible. Back up, sign out of every account, remove the device from your account, then reset. In that order; resetting first can leave it locked. Selling electronics.
Writing "working" without switching it on
If you have not powered it on, the word is untested, in the title.
Leaving your data on it
Photographs, messages, saved passwords, work accounts. A device you post is a device you no longer control. Account security.
Not saying what is included
Charger, cable, adapter, box, case. Every missing piece is money the buyer must spend before the thing works, and they will count it.
Promising battery health you have not checked
Say the age, say what the device reports, or say you do not know. A confident "battery is perfect" is the sentence that comes back.
Quoting a delivery price before asking about the battery
Couriers restrict lithium cells and the rules differ by carrier. Ask first, because the difference comes off your balance. Packing electronics.
Posting a damaged or swollen battery
Do not. Most carriers refuse it outright, and for good reason.
The one that costs most
Still signing out. It is cheap, it takes two minutes, and skipping it turns a good sale into a locked brick.
Questions people ask
Can I sell a device on instalments?
Not until you own it outright.
Is a replaced screen a problem?
Only if undeclared. Say who did it.
Do cables sell?
In lots. Alone, rarely.