activation lock

If a used iOS device has been 'Activation locked' then only the previous owner can unlock it, either by providing you with the account ID and password, or by carrying out the steps detailed in How to remove Activation Lock - Apple Support (as he should have done before selling it):

1. 1. Sign in to iCloud.com with their Apple ID.
   2. Go to Find My iPhone.
   3. Click All Devices at the top of the screen.
   4. Select the device that you want to remove from iCloud.
   5. Click Erase [device]. Select Next until the device has been erased. 
   6. Click Remove from Account.

After the previous owner has removed the device from their account, turn off the device and then turn it back on to begin the setup process.

If you are unable to contact him to do this then I'm afraid you will not be able to use the device - there is no other way of unlocking it at all. Apple cannot unlock it for you, nor find the previous owner for you, and neither can the websites that offer to unlock it for a fee: these are scams.

You can only unlock it with the ID and password used to lock it. The owner's email isn't necessarily his Apple ID. You need to get him to carry out the steps noted above or give you his ID and password. I'm afraid that failing that you will not be able to use the device.