Activate Iphone 6plus.

You need to contact the seller and ask him to remove this iPhone from his list of devices. You can also ask for a refund. Without previous owner, you won't be able to activate it. You've bought a useless brick.

Find My iPhone Activation Lock: Removing a device from a previous owner’s account - Apple Support

Well, there're two ways to remove the activation lock. You can do either reset current Apple ID's password signed in on device or gather "proof of purchase." and send to Apple.

FMIP will be removed from device and you can restore your device and setup as a new.