Authenticity cannot be settled on a video call — but in this one category, something better is available: ask them to turn it on and use it while you watch.
What to ask them to do on camera
Power it on. Then ask for something specific on screen: the settings page showing the model and storage, the battery health if it has one, the serial number screen.
Photograph the serial or IMEI, so you can look it up yourself before bidding.
Show the ports and the charging socket, close up.
Name which parts have been replaced. Ask it as "what has been repaired or replaced on this?" A screen or battery replacement is normal and disclosable; discovering it later is not.
Run it for a few minutes, if the price justifies asking. Things that fail usually fail warm.
What a refusal means
This is the category with the least excuse for refusal, because the item either works or it does not and the show is live.
"It was working when I packed it" is a sentence to be wary of, and so is a seller who has the device in the room and will not switch it on.
For a battery device, "battery is fine" demonstrated by a health reading is worth more than any adjective.
What this page will not do
It will not tell you how to identify counterfeit components or refurbished parts by sight. That is bench work, not camera work.
What it can tell you is that a powered-on demonstration plus a serial you looked up yourself is a far stronger position than any visual inspection of the casing.
What protects you if you get it wrong
A device that is not what it was described as is returnable for 7 days whatever the store's policy says, and a counterfeit is reportable.
Ask about delivery too: batteries are restricted by couriers, and a seller who does not know that is a seller whose parcel may be refused.
Questions people ask
Is refurbished the same as fake?
No. Refurbished is legitimate and has to be disclosed.
Should I buy a phone without seeing it powered on?
Not if you can ask, and in a live show you can.
Does a serial number prove it is genuine?
It is something you can check against the maker rather than against a blog, which makes it unusually useful here.
More: spotting a counterfeit before you bid, Metzee's position on counterfeits, reporting a seller, questions to ask before bidding on electronics.