They apparently preauthorize credit cards BEFORE confirming inventory. If the card you want is popular, inventory may disappear while in the middle of a transaction. I tried a couple of times before my bank called me since the bank's system detected fraud. My bank showed four transactions (plus three more which were declined once they froze the account). Bulk purchase is therefore useless.
Cardpool's response:
"The first four charges were cancelled and refunded when the orders did not go through. This usually happens if one or more cards is purchased by another customer before the order is completed. The refunded amounts will be reflected in your credit card statement within a few business days."