Question Steps before selling brand new AT&T Note 3

Hi guys,

could you please provide me a required steps I need to complete before selling a brand new Galaxy Note 3 (I've got with 2 years contract extention) on ebay/Amazon?

As far as I understand, I should register device with AT&T first, right?
Second, I should check model/firmware number to check ability get a sim-unlock (I understand, I do have a very low chances).

If you have an experience, could you please share?

Thanks a lot!