If the seller is reputable, then you should return this. No seller should be selling iPhones without a full factory wipe and full removal of the iCloud/Find My iPhone accounts. iPhones being sold in this state *can* be either lost or stolen phones that were never reported, though you can check the IMEI (should be printed on the back in tiny font) here. However, even if that comes up clean, it could just mean it was never reported as many people don't know how or forget as soon as they get a new phone.
As far as I know, there is no legitimate way to get around this.
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