I have limited experience of retail Point of Sale systems, but I think i have seen something that might be of use to you.
Barcode scanners that interact with excel are usually programmed to enter the code in the active cell and then "press enter" so to speak so that the next cell down is selected. I guess this is how yours is working too. The retailer using this had two workbooks, a sales sheet and an inventory and so long as they were both open at the same time the inventory was being updated on a live basis.
It used a macro that was attached to the "press enter" event. It would effectively look at the number that had just been entered and count the number of instances of it in the list. Then it would match the code with the list of codes in the inventory sheet. Predictably, the next step would be to subtract the countif result from the inventory number.
Is this the kind of Macro that you would need or am I getting the wrong end of the stick?