Bought an iPhone 7 plus but it is iCloud locked

If it's Activation Lock, please refer to: Find My iPhone Activation Lock

1) You will need the original proof of purchase for the device from when it was originally purchased.

2) You can recover the Apple ID + Password for the Apple ID associated w/ the device.

Don't get the OP's hopes up. For 1) that will only work if you are the original owner. And the OP clearly isn't. For 2) there is no way to recover the Apple ID or password if you are not the person who created them.

Unless the previous owner can remove the lock, it is useless to you. If possible, return it for a refund.