Apple ID

You sign them out by returning the phone to them and getting them to do it - or it can be done by the previous owner remotely:

How to remove Activation Lock - Apple Support

Remove Activation Lock on the web if a device is offline

1. Go to www.iCloud.com/find.
2. Sign in with your Apple ID and password.
3. At the top, click All Devices.
4. Select the device that you want to remove from iCloud.
5. Click Remove from Account.

What they should have done and failed to do is this: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Have the prior owner do the steps in the last link David McKinlay posted.

What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support