Bought a phone from Facebook but it’s locked

Only the person who marked it lost can remove the lock.

It is likely you were sold a stolen phone and have lost your money unless the seller will refund you.

You might have paid for it but it is apparently still owned - as far as Apple is concerned - by someone else.

Until that ”someone else” relinquishes control, it will remain unusable to you.

See:

How to remove Activation Lock - Apple Support

It will be of no use to you without the owners Apple ID and password. Stolen phones are sold on Facebook all the time and the ones that buy them will never get their money back. Contact Facebook to see if they are going to do anything about this problem.

"Bought a phone from Facebook but it’s locked: I bought a apple iPhone off Facebook and it says it’s reported lost but I paid for it how do I get it unlocked to use"

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Troubleshooting a Scam Sale:

Seems you were scammed, though a full-assumption on my part. So, contact FaceBook, seeing if the seller can be held accountable, and if you can get your money back. If this was purchased in-person, then always note that it is your responsibility to get in contact with them. Elsewise, it is useless to you.

Learn from this:

If this is a used device, then the person currently signed in to the iPhone needs to log themselves out of it. Elsewise, it is useless to your account. 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, iPad, or iPod touch - Apple Support. The seller should have known better. So, return it, requesting that they log out of it.