There's opencart POS extension : [url removed, login to view]
I need this one to be modified to have "offline" and "online" sync using html5 application cache and for database sync, maybe it's using pouchdb when offline or no internet connection.
If you have a question, just ask.
Do not rush, test it first and know if you can do it or not.
Please type "POS Mod" so I know that you're not a bot.