To leave feedback for others, the easiest thing to do if they haven’t pinned their feedback post to their profile (see below for example) is to type this into the search bar:
in:title #buy-sell-trade:traders-feedback <username> , so for example if you wanted to search for Ben’s feedback thread, you’d search:
in:title #buy-sell-trade:traders-feedback benmichael86
once you’ve found their thread, just add a reply w/ your feedback from your transaction with them.
If they don’t yet have a thread, you can start one for them in the same section, being sure to put their username in the title so it’s easily found down the road
This post is a good overview of how things are currently done w/ some screenshots