Regarding Deep Discount: They do have a hit and miss record when it comes to honoring mis-prices. They have done so in the past, such as the re-release of Familiar of Zero. But if they choose to not do so, through their mistake or receiving incorrect information from a vendor, they simply cancel the order. They do not charge you the higher price.
From their policy regarding errors and omissions:
In the event a product is listed at an incorrect price due to a display or typographical error, or an error in pricing information received from our suppliers or store partners, DeepDiscount.com shall have the right to refuse or cancel any orders placed for product listed at the incorrect price. DeepDiscount.com shall have the right to refuse or cancel any such orders whether or not the order has been confirmed and your credit card charged. If your credit card has already been charged for the purchase and your order is cancelled, DeepDiscount.com shall immediately issue a credit to your credit card account in the amount of your original charge.
DeepDiscount.com makes every effort to provide accurate and current information concerning all products. However, information errors and omissions may occur in the data entry or transfer process or as a result of information provided from our suppliers, store partners, or other third parties, and except as set forth in this policy statement, DeepDiscount.com shall have no liability for any such errors or omissions.
But with prices these good, it's probably worth taking the chance. If they don't honor the price and cancel your order, you're out nothing. If they honor it, you're saving quite a little pile of cash.