Something similar was a recent conversation in the Code Co-op. It's on the list of to-do's to make it, but that list is very long and regularly gets interrupted by other things that need attention.
Alternative option (perhaps less work as most of these pieces of the jigsaw puzzle already exist) would be to get customer to subscribe to product_notifications on the (out of stock) product. But have notifications linked into Queued E-mails to make a complete hands off system.
So, stock = 0 (or lower?)
1. in product listings, change buy button to something else (or remove it?)
2. in product page replace buy button with `let me know when in stock` -> this adds customer to notifications for that product
When shopowner increases stock
3. auto schedule QE mail to all customers who want to be notified about the product
There are many ways to approach this.
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The simplest way, I think, would be;
1. Install Notifications module, I think it's now an Addon.
2. Put it near the Buy Button
3. Override both of these TPL files to show (or Not) based on "stock <= 0"
At this point you have two modules swapping in and out, one allow buy is in stock, one allow signup if OOS. So far, so good.
Now override the product_card component in a similar way, to remove the buy button based on stock <= 0.
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Another, better way;
build a product_info PI layout module that does the job itself. This new Module swaps button<->subscribe based on Stock. Install this. Uninstall the usual buy button. More self contained, less chance of forgetting you have over-ridden TPL files.
In the Product Card component, instead of overriding it, hook into it to manipulate the Buttons.
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In either case, the `quick way` or the `better way`, once this is working... then find and install the Notifications Scheduler so that when you increase stock of the Product...Emails are automatically created and scheduled to go to those who signed up for notifications. You now have a complete, hands off system.