Click on your username at the very top of the forum, then in the left bar under your profile pic click on "find latest posts". If you posted in the classifieds
thread, you'll be able to find it this way

From there, there is a button you can click to leave a seller feedback at the bottom of the first post in their FS thread (a little green icon near the left side of the bottom of the post). Clicking this button will auto-fill the thread url in the feedback form, makes things a little easier
I always make a point of subscribing to a thread that I
buy or sell in, that way I can find it again easily (even after it has been moved to the closed threads section) in order to leave feedback.