Is it safe to buy a new (but opened) iphone from ebay/Amazon?

I've seen these iPhones where they'll say they where opened to check for package contents, display model, etc…
Are these just done purposely so the seller can blacklist it a few days later, or are they genuine.

Think about it - if the phone were new, not a Chinese knock off or not stolen, they would be able to just sell it back to where they bought it from, even if it was open, as long as they had the receipt