For those unfamiliar with GMG, they have a peculiar checkout procedure where a 15 minute countdown starts when you add something to your cart. This is supposedly to reserve a copy for you so you don't have to worry about it selling out while you're paying for the item. If you manage to get something in your cart, you're guaranteed to get it...as long as you complete the checkout within 15 minutes. If you don't, the order gets cancelled and the item is removed from your cart.
Given the current state of the site, it's pretty much impossible complete the checkout in 15 minutes. So even if you get it into your cart, don't expect it to stay there.