JacobsParts is an eBay seller, and in their eBay auctions they clearly state that it's a third party drive, not a Microsoft item, in a "compatible" casing. This seems to be the going rate on eBay for such drives. I think it's also clear from their eBay ads that they don't include the transfer cable, unlike the Microsoft kits - you have to purchase it separately.
That said, it's hard to see that this matters much. Their eBay ads also state a 30 day return and 6 month warranty, so it seems safe enough.