I don't think you can add it to your cart unless it's available at the time... then if it sells it out'll stay in your cart but say:
"This item is no longer available from the seller you selected."
however if it comes back in stock and you refresh your cart you'll get a notification.
Yeah that's right. I just put the item in my wishlist and watch from there, if I can't add to my cart because Amazon has sold out.