Activation locked

If you're able to contact the seller, ask them to unlock it or give you a refund. If not, ask OfferUp for a refund.

The activation lock is an anti-theft measure. You won't be able to unlock it yourself, and Apple won't do that for you.

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Here are the Apple support pages that outline the Activation Lock - and the only mechanisms by which it can be turned off:

https://support.apple.com/en-us/HT201365

https://support.apple.com/en-gb/HT201441

As you will discover, unless you are the owner of the AppleID that was last used to activate the device, or unless you can secure the cooperation of the previous owner, you’ll not be able to defeat or bypass the Activation Lock. Without documentary evidence that you are the original owner (such as sales receipt showing the serial number), Apple won’t assist - and nobody here will be able to help you.

The All Too Common SAD Reality of Buying … - Apple Community