Dad just bought a fully working iPhone 16 from Facebook, but I can't find the scam

My dad just told me he was driving home having just bought an iPhone 16 Pro Max off facebook for £800 (retail is £1599 here). He went to the guys house, checked the phone, tried to make a bank transfer but there was no phone signal, so the guy let my dad go home, WITH THE PHONE, to make payment when he got home.

I've just been round to dads house and all looks good. Not signed into iCloud, all clear on IMEIPro, and then the seller called to ask if he'd made payment yet. He even has a digital receipt from a major phone provider for the phone and the name and address match (though the whole thing could be easily made at home).

This has to be a scam, no one buys a thing and sells it for half price a month later, but I can't see how. I would rather lose out on a good deal than lose out of £800, but he disagrees. Someone please, tell me how my dad is about to scammed before he makes the payment.

UPDATE: It's a fake iphone. As a young tech savvy guy, I was fooled, but the camera only zooms 3x and Podcast app asks for a Google account.

FINAL UPDATE: Dad met the seller in a public place, gave the phone back telling him it was a fake, and he said he would take it back to the shop. Dad offered to buy it when they shop gave him a real one 🤣 If i had to guess, seller got scammed himself buying this phone, and was trying to recover his loss.

Thanks all, you are the true heroes of the internet