3dcart's Waiting List feature allows shoppers to add themselves to a Waiting List if/when an item's stock reaches 0.
Rather than the usual "Add to Cart" button, the shoppper would see a "Put me on the Waiting List" button and a pop up window would allow them to enter their contact information.
When the item's stock is replenished, the contact information entered can be used to email the customer to let them know the item is again available. The notification process can either be done manually by you or through an automated script.
The page found by going to Products =>Waiting List will show you a list of items that have waiting customers as well as stock information for the items. From this page, you will be able to email the customers to let them know of the items re-availability.
For instructions on using the Waiting List feature, please review our full tutorial located here.