Two questions prevent nearly every bad outcome here, and most buyers only ask one.
The questions
"Can you turn it on?" For a phone or tablet, the settings screen with the model and storage. For a laptop, the desktop. For audio, a sound. Non-answer: "It was working fine."
"Have you signed out of your accounts and removed it from your account?" This is the one that saves the sale. A device still linked to its previous owner's account can be impossible for you to set up — not difficult, impossible, and no reset at your end fixes it.
A seller who knows what you mean answers in one sentence. A seller who does not probably has not done it. Selling electronics.
"What is included?" Charger, cable, box, case. Every missing piece is money you will spend.
"How old is the battery, and what does the device report?" "I do not know" is fine. "Battery is perfect" about an old device is not.
"Has anything been repaired?" Screen, battery, anything opened, and by whom.
"Any dead pixels or burn-in?" Ask for a white screen and a black one. Different faults show on each.
What protection covers
A device sold as working that does not work, one that arrives locked to somebody else's account, missing items that were listed, or a different model: seven days from delivery.
Questions people ask
What if it arrives locked?
That is not as described. Raise it straight away.
Is untested worth bidding on?
At untested prices, yes. The honest listing says so in the title.
Should I ask for the serial number?
You can. Think about whether you want it published in a public question.