Seriously? The seller pulled the "I'm a retired Marine" card over whether he'd sell a fake jersey or not? That's great & all, but so irrelevant.
It's definitely fake and I don't see how he could deny this given the blatantly obvious blue stripes missing from the jersey.
EDIT: There's a slight chance it may be real. I notice you bought a YOUTH version, correct? Sometimes in addition to the official ones, the youth jerseys come in different color combos (and are still authentic). Anyway, if I had to bet, I'd still lean towards fake, since I can't find a single official retailer online to sell a jersey in that color combo (white without blue stripes).