The more important question is: How do you know it is new in box? How do you know it's not a 1 year old machine still in the box (in which case there would be no Apple warranty left)
How do you know you will get a machine at all once you have sent the money?
Apple notebooks are among the most common fraudulent auction items. They are desirable and are seldom if ever sold on a discount.
If you are looking at a 'new' machine that is discounted 20% or more below the AppleStore price, that means it is probably not new, or it is stolen, or it is a fraud.
Guide: Hardware :
Buying Online