Hiya
I'm confused by your post re barcodes, yet intruiged. The idea suggested on the posting about plumbing it all together no doubt fills you with fear, but it is possible to have a basic scanning facility and the only costs being a relatively cheap scanning device.
Effectively it acts as an alternative to a keyboard, it scans the barcode, if succesful it takes one action, if not succesful it asks for a rescan or a cancel.
Ive used the same system before for a cd warehouse issuing stock to clients, or on consignment to shops. Its not such a big deal, but its all down to at what point you want the stock to be depleted on the computer, and how you want this to interact with the customer billing (assuming its billings oriented, and not just goods received)
A decent scanner costs about £50 from maplins, and as said above its merely a keyboard under another guise.