I like to think about Ubercart Inventory as the "View" and "Controller" in a "Model-View-Controller" architecture. The "Model" is the data store where stock levels are kept, along with the functions that interface to the data store. For example, the core uc_stock module could be used as a model which keeps the stock data in the Drupal DB. A new module, for example uc_quickbooks, could provide a model which keeps the data in QuickBooks and accesses it from UC using the QB SDK. UC Inventory could then use *either* model, or *both* at the same time. UC Inventory would contain admin pages and forms which let the admin check/set stock levels (this would communicate with QB to find out or change the data stored in QB), and would provide user interface components to notify the customer about available stock, notify the admin if stock falls below a threshold, and optionally prevent the customer from purchasing more than is available.

The goal is an extensible architecture where the actual stock database can be on any system, either within Drupal or on an external computer, while still allowing your Ubercart store to limit on-line purchases and giving UC admins the ability to see inventory reports.

Based on a brief look at the QB SDK documentation, QB has a web services interface, so I think this would be easy to do.