For products that go out of stock, your trade customers can add them to their OUT OF STOCK / BACK IN STOCK LIST, and then be notified when the products are back in stock.
They will see the following in their menu bar and the number in green will be highlighted when products come back into stock.
Viewing Email Alerts Signups
When you are logged in as web admin, you can click on Orders - Out of Stock Alerts to see all email alert sign-ups in the last 3 months. They are sorted with the products with the highest potential revenue at the top of the list.
Sending Email Alerts
You can either send out email alerts manually or you can have this automated for you.
Full details on both these options are available here: