One of the reasons I'm asking is that, as I rewrite the Ubercart Inventory API module, I'm thinking about how to make the code flexible enough to allow extensions that meet many needs. For instance, if Quick Books has an API available for an external application to use to get/set stock levels stored in the QB database (does it?), then the Ubercart Inventory API could easily allow you to keep all your inventory in Quick Books while at the same time present real-time inventory status to the customer (e.g., disable add-to-cart button if out of stock) and to the admin (e.g. show reports of current stock for all products).

Alternatively, the Ubercart Inventory API could provide a method to allow external programs like Quick Books to get/set stock levels stored in the UC DB. Assuming of course that QB can be set up to use an external DB for stock control (can it?).

Maybe the UC Inventory API could even allow for both ways of doing things... Hmm...