For Sales items: The item is not actually reserved for you until you actually place the order. The inventory status on the site is up to the second real time, but the item is not reserved for you until you complete your order.
For Rental & Used Items: If the item is in your cart it is yours, unless you leave the site for 5 minutes and you are holding the last copy of something.
I hope this answers your question,
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XRentGuy

