A seller would just about have to open it to verify what he is selling. For one thing, he'd have to look at it to check condition. For another, if the seller himself didn't look at it, the buyer could claim he got a box of bricks and wants his money back.
For vintage items stored in original boxes, climate is an important issue. Unless it is some kind of military equipment sealed in moisture-proof wrappers, items stored in cardboard boxes in humid climates can incur moisture damage. Cardboard absorbs moisture; when it evaporates, the moisture is released and can condense on objects stored within. "New in original box" isn't always a good thing. Good thing in AZ; bad thing in FL.