When a product is out of stock you can either lose a potential customer or convert a lead.
If you want to make money you can use an email form to gather leads by offering:
- pre-orders;
- alerts for when a product is back in stock.
Leverage one of these methods to get your visitors' email and you will be provided with your opportunity to make the sale when your product is available.
Additionally, if you collect a potential customers' email - you can help and upsell the lead while they wait - providing you with another chance to make a sale.