Quantity on hand?

We have kind of hit a stumbling point in our set up and i'm hoping someone can help us out. We are a broker which means we do not own the things we sell, however, there are a finite number of units available. For example, lets say someone wants to sell 8 speakers - they would list those 8 speakers on GearSource. We would show a quantity of 8 available and could not sell more than 8.

Currently, as far as we can see, the only way NS can deal with that is to put 8 "In Stock" - then sell only the stock available. The problem is showing items in stock would mess our books up completely. When we signed up we were told that we could use "Stock Available" the same way as we would "Stock on hand" - and have it work in the same way - but show 0 in stock and use drop ship PO's etc to order. But now that we're trying to figure out how we have hit a brick