How do I unlock A device I bought from someone else.

I'm afraid you don't. 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).

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 should if possible return it and ask for a refund as in this event the device is completely useless.