iphone scams on donedeal

went shopping for a new iphone on donedeal. I noticed a few that were to the top of the range, a few weeks old, unwanted present, still in the plastic, "unlocked to all networks" - and close to half the price of a new one.

Surely a scam - right?
I'm curious to know how it works.
I ran one seller and he said as it was a present he had no receipt but it was bought it from carphone warehouse as a prepay phone - that's why it was unlocked. I rang the carphone warehouse and they didn't sell the 64gb model that the buyer was selling, and advised me that no one apart from apple themselves sold unlocked new phones.

I've since noticed the phrase 'factory unlocked by Apple Store' is common in iphone adverts. I'm suspicious of these too.

How to you suppose the scam works, do they buy under contract and then dont honour the contract?. Surely its not a Chinese copy, I'm pretty sure I'd be able to spot one on inspection as would anyone that owned an iphone previously. ??