This was not “brand new in the box” as you describe.  The fact that it had MDM installed/enabled means it was was taken out and at a minimum, configured for use.  The moment that happens, it’s no longer new but used (pre owned). 

The problem here is that you could have something that is legitimately sold from Verizon to the third party and the MDM was innocently overlooked to it being a stolen device in which case, if you are in possession of it, you could have it confiscated by authorities without compensation to you. 

In this case, Verizon being a large company with a large bureaucracy it will be a Herculean effort to validate its legitimate and to get MDM removed.  I would return this to the person you purchased this from for a full refund as it’s not as described - brand new. 

This is not to say that buying from 3rd parties is to be discouraged; you just have to be more diligent. Here are some tips for dealing with 3rd parties:

* use a service that offers purchase protection like PayPal or eBay
* test it out; boot it up and test before closing the deal. If you run into this, walk away. 
* boot it from a completely off state.  If the machine is running and “waiting” for you, it could be a sign of trying to obfuscate issues. Shut it down completely and boot from off. 
* test everything while on battery.  There are keyboard test sites or you can use Karabiner Elements (it has an event viewer) this will help you ensure the keyboard is good. 
* check the battery stats (link coming).  You can do this from Terminal without extra software. 

Most of all, live by the philosophy caveat emptor or buyer beware. If a deal seems too good to be true, it probably is.