Do you have a label printer already? The 'Brother' brand label printers come with software for making labels that can make standard UPC style barcodes.
As for the inventory tracking... You can easily track inventory with a spreadsheet manually, however, automatically updating that spreadsheet via a barcode scanner would require EXTREMELY advanced scripting (if it's even possible to do). What you need for keeping track of sales and error catching like duplicate entries is a "Relational Database" like Filemaker Pro, Microsoft Access, Oracle, or the very good and FREE Open Office Base.
I use Filemaker Pro 11, which is fantastic and powerful, but fairly expensive and complex. They do run sales on it once or twice a year though. With this kind of program, you can create rules for the database so it won't let you create duplicate entries (called 'data validation'), and they usually support barcode scanners. You can also set up automatic reports so that at tax time, all you have to do is click a button and hit print. All of this needs to be set up by a programmer though.
Take a look at these and see if this helps:
http://www.filemaker.com/products/filemaker-pro/http://office.microsoft.com/en-us/access/http://www.openoffice.org/why/why_great.htmlHope this helps, best of luck!
-Dan