So, unless Amazon has changed things this is how the waiting list works:
Once a customer places an item in their cart it is reserved for them. Meaning, the customer can take their precious time checking out and nobody else can purchase it from underneath them in the meantime. However, there is a time limit an item can be placed in a cart without being checked out, somewhere around 10-15 minutes. After this time limit the item is automatically removed from the customer's cart and placed back into open circulation.
Got that? Good.
Well... If an item "sells out" it might really not be sold out, but rather just placed into people's carts. So, after initial stock is depleted new customers are placed onto a waiting list. If someone doesn't purchase the item after placing it into their cart it is automatically removed and given to the next person on the waiting list. This new person can decide if they still want the item by allowing them to place it into their cart or once again be moved onto the next person in line.
You would be surprised at the number of people that quickly place item in their cart but never end up purchasing. You can be person #100+ on the waiting list and depending on the item stand a good chance at still getting it.