You'll need to create a user for your sales rep first, this will help you track your sales rep's sales. Then, deactivate that user.
When creating a new Sales Order, select the Sales rep you created to mark the sales from them, you'll see them with Deactivated text. Process your Sales Order normally.
To see how much your sales rep has made, go to Sales Report, select the Sales rep name from the drop-down list, click View, done. :)