By default, your wholesale customers can add items that are out-of-stock to their cart. This option is useful when you know that your out-of-stock products will likely be available before your customer makes payment.
If you do not want your customers to submit orders that contain out-of-stock products, then you can disable Out-of-stock items on the Preferences page in the wholesale channel. If you disable this option, then all out-of-stock products remain visible in the wholesale store, but your customers can not add it to their carts.
It's possible that a product will be in-stock when a wholesale customer adds it to their order, but out-of-stock when they try to pay for it. This can happen even if you prevent your customers from adding out-of-stock products to their cart and you reserve inventory when you reserve their draft order. For example, a product can be in-stock when your customer submits a draft order, but be sold out when they open their invoice to pay for it.
Completing orders that include out-of-stock products
If your wholesale customers pay for their orders at checkout and you track inventory using Shopify, then your inventory is checked before your customer can pay for the invoice. When your inventory doesn't contain enough product to fulfill the order, the following events occur:
If the product is in-stock, but the amount available is less than what the customer ordered, then your customer's order changes so that the number of items ordered matches your inventory. For example, your customer orders 10 fidget spinners, but when they are ready to make payment, only 5 exist in your inventory. Their order changes automatically so that your customer orders and pays for 5 fidget spinners.
If the product is out-of-stock, then the customer is instructed to contact you, the merchant. They cannot pay for this order or change it. You must open their draft order in the Shopify admin, remove the out-of-stock products, and resend the invoice.
When you accept your customer's payment (by clicking Pay with credit card on the draft order in your Shopify admin), then the inventory check occurs after the payment completes. This means that you can accept payment for products that are not in stock yet. If any product is out-of-stock or there isn't enough in stock to fulfill the order, then a warning appears at the top of the order in your Shopify admin.