Here’s a writeup of the current state of affairs
Upon transaction with a user, either find the existing topic for that user in the Traders Feedback category, or create a new one if it does not yet exist. Search by the username in that category:
After a transaction, post your feedback on the transaction as a reply to the seller’s feedback topic , like so:
To make it easier to find, the seller attaches that topic to their user card via their user preferences, under User → Preferences → Profile → Featured Topic near the bottom
At that point, you can now easily find and refer to the seller’s feedback topic on their user card, like so:
There could certainly be more sophisticated ways to do this, but this simple method leverages the built in functionality in Discourse and it is relatively easy to understand… it’s just another reply in a standard topic.