There's no indication even in the OP that it's stolen good. If the "silver bag" has his nano SIM, it's perfectly understandable that seller would want to keep it if he bought it on his own. When you sell iPhones, you always take your sim out and keep it.
A new sealed iphone being sold via craigslist..... with a silver tab being pulled off (the door alarm sensor I'd imagine)....
Not stolen? Yeah right.