In regard to the serial number, if the seller supplies it before the sale is final, you can go to www.nikonusa.com, click on support then on products and you can enter in a serial number as if you are registering a camera. The site will let you know if it is a USA model or not.
Through a slight personal experience, it appears that non-USA models have a serial number that start with a 4. USA models start with a 3. (Maybe someone else can verify this)