You guys can leave actual feedback for each other here on the site :
Click on the number in brackets where it says Trade Count beside your trading partner's avatar next to one of their posts in this thread, for example...
...then, on the next page, click on the Submit Feedback for (trading partner's member name) link and fill out the form. Then, once you're finished, hit the Submit Feedback button.
When you're done, your trading partner will have a number (one higher than it was before, 1 in both your cases) in the brackets after Trade Count beside their avatar, and other users can click on the number to see how others rate that user's trades/sales.