Need Guidance on Consignment Selling or Customer-Owned Inventory Best Practices

We are struggling with how to manage customer-owned inventory on several of the eStores we run for our customers. If anyone deals with customer-owned inventory or consignment selling, and is willing to share their practices and procedures, it would be most appreciated.

Specifically, how we would like the flow to go, using a hypothetical Item A, is that we purchase a quantity of Item A for the Customer's eStore and sell it to the top-level customer, recognizing the revenue. That part's fine, but then we don't actually ship out the stock of Item A to the top-level customer. Instead, we hold the stock and when either the customer's employees redeem their budgets on the customer's estore, or they pay by credit card on the estore, we ship the stock to the end user. If the estore sale was by