Help Setting Up Inventory Sheet to be used with a Barcode Scanner

Greetings All,

I've seen several forums on here addressing the problem in various ways and they've been most helpful, but I've reached a block with a project that I am working on. I work in a small computer repair center for a larger organization. Throughout the day several workers form different departments may remove things from our small equipment inventory and we're looking for a simple and cheap way to track the quantity of each item. My goal was to have the following

1. Scan a SKU and if it already exists in inventory it will deduct 1 from the total count
2. If the item does not exists it prompts you to set the Product Description and a starting count for the quantity

I've managed to get goal number working with the help of some other forums, but I cannot for the life of me figure out how to do step 2. With how I currently set it up it adds the value, but overrides cell A2 and then copies the information from B2 into the next empty cell in column B.

I've attached a copy of my spreadsheet below. Any help would be greatly appreciated.