Basically you are calling microsoft, saying your xbox is bad (most likely with a cd problem) and you are using a serial number that is either randomly generated or there is a generator out there. You then and give a shipping address you will need to provide a credit card, of which you will use a prepaid one or something similar.
Microsoft replaces the xbox and is out of the money.
Here is the ramifications on this:
1) nothing happens and you make money
2) They figure out where you are, and you are arrested, selling stolen goods or even fraud
3) They know the address that it is being shipped so guess who gets a bill, when the credit card doesn't work.
+1 I just typed virtually the same response as you and was debating whether to risk calling someone out, being a newb.
Basically you're setting up a advanced exchange repair..
"Advanced exchange: Microsoft will ship a replacement product to you first, and then you'll send back your non-working item. There are a few important things to note about advanced exchange:
You must provide credit card details for an advanced exchange. A hold will be placed on your credit card for the price of an Xbox One console or Kinect sensor.
You're responsible for sending the non-working product in within 14 days after receiving the replacement product to avoid having your credit card charged."
The trick is Microsoft don't put a hold for the full cost of a console on your card, they just do a check. So people use a pre paid visa or vcc with £1 on to do this and the serial numbers to set up a repair can easily be found or given to you if you ask a seller on eBay etc.
When Microsoft clicks on they're not receiving the broken xbox that doesn't exist back they will try charging the card which obviously won't work where it only ever had a pound on it and the only trace is the delivery address which conveniently is the buyers and not the seller.
With the use of Bitcoin as the payment method and not accepting PayPal etc (there is nothing identifying the seller), but there's nothing stopping Microsoft chasing it up or bricking your console down the line or worst case scenario the seller using stolen credit cards to pass the security checks (Microsoft are meant to be clamping down on pre paid cards) which would result in more than a bricked console for whoever buys one.
Well because both of you said this, I have to close this.
But I have to say this, there are many people doing this and as I know so far no one got caught. No one ever gets billed from Microsoft. Microsoft is a big company, if they would want to stop their loss, they would already make some changes in their policy. But thank you the heads up.