1, First you need to navigate to the sales thread of the user you dealt with, or any post by the user. Once you are viewing a post by the user click on their feedback score under their name.
2, Click on the 'Submit Feedback' link.
3, Fill in the feedback for the user and click the 'Submit Reply' button. Adding a link to the sale in question is a good way of keeping track of sales.
4, All done, you should now see your feedback on their iTrader profile.
Thanks to LeeBristolAirsoft for doing some leg work on this.