Rippley_Siyh9 Saad
"What can I do to unlock an iPhone with an Apple ID I don't know?"
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From a more general perspective...
Getting Into a Locked Device:
A. Reporting this:
Seems you'v been scammed, though a full assumption, on my part. So, if the seller will not take this back, then you're out of luck. Maybe contact local authorities. If purchased off of a Web site(i.e. eBay or Amazon), then contact the Web site, seeing if the seller can be held accountable, and that you get your money back.
B. How to Properly Sell a Device:
This is the site the seller should have used prior to selling it to you: What to do Before you Sell, Give Away, or Trade In your iPhone or iPad - Apple Support. They would perform as instructed, including the iCloud logout. The seller should have known better.
C. Learn from This:
So, take this as a learning experience, knowing to always go there, prior to making any purchases, whatsoever. Ask the seller to let you log into it, prior to handing in the money. If the seller decides to disabuse by those rules you've set froth, then do not proceed with the transaction.
D. If Purchased Off of a Website (i.e. eBay):
Contact the site, seeing if the seller can be held accountable and if you can get your money back.