What are differences between buying an iPhone from Amazon or from Apple store?

What are differences between buying an iPhone from Amazon (brand new) or from the online store?

I just want to know the differences between buying from an official site or an online store like Amazon, for example I want to buy Microsoft Surface Pro2, and I'm outside the United States decided that I want to buy a product from the US via online a shipment let's say. Do you guys preferably choose the official vendor or an online store? What differences it makes?

I'm sorry if I confused you, I just wanted to buy Surface Pro2 its NOT available in my country and never bought something from the Internet, I need your expertise please!

I would get it from the Apple store, thats just my opinion.