Jayjojo Said:
"What happens if you buy an iPhone that’s still under warranty but person that sold you the phone lost or doesn’t have proof of purchase and you are in a region that Apple doesn’t have an approved dealer or service provider for its warranties??"
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Then you are out of luck, and have been scammed.
Troubleshooting This:
Seems you were scammed. So, contact this seller, asking that they take it back and log them seleves out. If they do not log themselves out, then there is nothing you can do about it.
Where was this Purchased?:
If this was purchased off of a Web site (i.e. eBay), then contact the Web site, and see if the seller can be held accountable for this, and if you can get your money back.
Your Own Terms & Conditions:
Next time you go about purchasing such an item, do it in person, and have the seller go here***. Then, request that you log in, prior to handing over the money. If the seller dis-abides by those Terms & Conditions you've set forth, then do not proceed with the transaction.
***What to do Before you Sell, Give Away, or Trade in your iPhone, iPad, or iPod touch - Apple Support