In my experience, motherboard boxes don't come sealed, just (some times) the anti-static bag inside (as well as whatever driver disc and accessory/cable bags are included). I believe this is for retail purposes, so that customers can actually see the board layout and ports before buying without breaking any seals, given that the boxes generally only give a vague overview (and unlike a GPU or other components, the physical layout and I/O is complex, highly variable, and really matters). Unless the anti-static bag has a broken seal on it, I wouldn't worry at all.