Getting a new phone and not being able to use it is a bummer, Thelostones2014. If the apple id is still connected to it you will most like have to reach out to the seller. You could try to restore through itunes to see if that will clear it out. Pull up itunes on computer> power phone off> hold home button while plugging into computer and wait for itunes symbol to show on phone or itunes to say phone found in recovery mode.
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You will need to contact the original owner. They need to remove the iPhone from their iCloud account and Find My iPhone. They can do this remotely. Here is the link with steps just in case they don't know what to do. https://support.apple.com/en-us/HT201351