I strongly disagree with limiting access to the shop in any way. Punishing people for being long time users or frequent visitors sounds like a terribly counterproductive thing to do.
What I would rather suggest is to stagger the release of keys. Put half the keys up on the store then in 8 or so hours release the rest of the keys. This is a tactic that many concerts and cons use to ensure that more people have the opportunity to purchase. This would also alleviate the time zone troubles a little.