Since QBO does not have a Rep field for sales, usually you create a class or location to use for this kind of thing

class allows you to tag each item line in a transaction
location allows you to tag the entire transaction

From what you say, location would be the best to use

then you can run a P&L by location, set the P&L report to cash basis and it will only show paid invoices, or run a location report and see what you can do in report customization