backorder functionality is not yet available in bigcommerce. However we can use some apps to track backorder from bigcommmerce marketplace. GritGlobal is one of the app to track backorders. But to keep in mind when using GritGlobal app is , You should not track inventory for the products which you want to accept backorder.Select the product and assign backorder for the product. you can also import products.
GritGlobal is an example.
https://www.bigcommerce.com/apps/backorder/
App Specifications
Rescue Out-of-Stock Revenue
With BackOrder, you can generate more sales through customers you would otherwise have lost due to stockout.
Enable BackOrder Automatically
Assign your products to the app and switch to backorder instantly when inventory hits 0.
Save yourself from the hassle of workarounds like using pre-order, or editing every single product’s availability text.
Flexibly apply BackOrder to both products & variants
BackOrder can be applied for specific products/variants (different sizes/colors/materials/etc.)
Manage Storefront Experience
Customize Backorder button, product message, packing slip, cart warning and show back in stock date.
Backorder Inventory planning
Set a BackOrder Threshold to control BackOrder quantity. At the same time, plan your incoming inventory and Back in Stock Date.
Works seamlessly with your IMS/ERP
Your IMS/ERP is still the single source of truth when it comes to inventory management.
Any updates from your IMS/ERP to BigCommerce will also be synchronized properly to BackOrder.