I am 99% sure that It's got RROD and now he is afraid,,,,,,lool ,
I know cause my friend did something similar to get rid of his refurb 360 after MS sent it back to him.(he got a 2-months old receipt from one of his friends and sold his 7 and half months old 360 to get a PS3)I know he is quite an evil.
There's another way you can do that, say you have a 18 month old X360 you wanna sell, buy a new 360 at Target or Best Buy and make sure you use a Credit Card, do not open that 360 and go back to said Best Buy or Target a couple hours later or the next day and return it (Do not open the 360 box), say you lost your receipt and they'll ask you if you paid by Credit card, say yes, they'll then ask for your credit card, look up the transaction and issue you a refund.
Perfect, now you have a new receipt for your 18 month 360, wait a couple of weeks or a month so as to not look so obvious and craiglist it. Note, this little scam also works with most other electronics you might want to sell to unsuspecting people, I do not condone this kind of behavior.