If you put something in your cart, it's not truly reserved until you complete the purchase. So if there is one thing you really want, buy it first and then go back for another order. If something sells out while it's in your cart, either it will get removed automatically or an error will pop up.
Some people have had luck placing items in their carts ahead of time and just waiting for the prices to change, but that doesn't always work for a variety of reasons.
And of course, make sure everything is correct in your account - addresses, credit card info, etc...