I work at a bookstore and we often have "Strict On Sale" releases. They're scanned into our inventory, so our customers can see that they're in stock when they do a search on our computers, but the books are in the back in sealed boxes until the release date.
However, I firmly believe that I should be there for the customer, not for the company. Our head manager frequently bends the rules to make customers happy. I just don't see why I should feel any affinity for a company that (at the top) sees me as little more than a number. I'd much rather make another human being happy.
That said, would I sell a customer an early copy of the next Harry Potter book? I don't know...probably not. I'm pretty sure a Wal-Mart did that out west for the Deathly Hallows and the store disappeared clean off the face of the earth (most likely by means of the dark arts).
If I were sure I could get away with it, though, then I'd probably go for it. Imagine how excited that customer would be! They'd probably be excited enough to come back the next day and tell me Snape kills Dumbledore!