Thanks for that - it's somewhat reassuring. The reason why a red flag went up is because whenever I've bought a motherboard, either the box OR the anti-static bag OR both were sealed in some fashion. So, when I saw that none of those were sealed I thought something might be up.
I am super, super anal about stuff that I buy. If I had even the slightest concern that my motherboard was an open-box product, I would have returned it. In this instance, I was never worried. In my experience, I have never had a motherboard box come sealed. As for the anti-static bag, I really can't remember if it was sealed or not, but it certainly was "crisp" (no bends or wrinkles to suggest it had been used and then re-stuffed). Unless you had any secondary-indicators, I wouldn't worry.