Supply chain issues are critical to large-scale e-commerce, especially if trying to maintain just-in-time inventory or heavily reliant on drop-shipping.
It is important to be receiving periodic updates on supplier inventory and price change so that when an item goes out of stock it can be immediately removed from appropriate sales channels, and if the item comes back into stock or the stock becomes limited, that the sales channels reflect that.
Once an order is placed, either a drop-ship request has to be sent to the supplier or a stock order has to be replaced, but in either case tracking numbers are needed to track shipments.
Availability of items needs to be current in near real-time so that commitments aren't made that can't meet customer expectations.
Supplier invoices need to be reconciled to assure accurate cost accounting.
In a best case scenario a supplier will have an API that allows direct queries to their inventory and ordering system.