Not open source, but you might have this already. How about just scanning everything into a spreadsheet like Microsoft Excel?
I'm only half kidding. We used this approach for a one time inventory cut-over.
You can set most of the handheld USB barcode scanners to add an Enter after each scan (go to the next cell in spreadsheet).
Then you ...
I can also recommend Tabshop mentioned in comments above.
I use it for a mobile shop for a couple of months now. See the link for full description. it offers sales, inventory, barcode scanning, etc. It is also very easy to use and setup.
The only thing it does not have is your "employee/attendance" requirement.
In future, if you want to sync with the cloud,...