I think you are misinterpreting the meaning of "Buy Now". All this means is that you can place your order now, it will not be shipped until the store receives the item in question. In other words, it's an advanced order, it doesn't mean that it gets shipped before the release date. If you want it, placing such an order isn't a bad thing because any decent on-line store will not charge for the item before it actually is shipped.
And BTW, while Robert's Anime Corner Store is a nice on-line store to browse, I find it's a bit expensive, for DVDs I prefer somewhere like
DVD Pacific. But that's just my preference
DVD Pacific list
Get Backers as Pre Order,
vol 1 release date as 24th Aug (as I'm sure most places do).