As the seller, go to your original FOR SALE thread, right click the page ID to highlight and copy.
Presumably the buyer has a post in this thread or at least a PM. Regardless, once you find anything with their online name, (even a PM) left click their name and go to the bottom of the area with feedback info.
It will look like this:
https://www.rccanada.ca/rccforum/member.php?u=26927
You will see a dark posted line about viewing the feedback info for any member. When there you will see the option to leave feedback.
The entries are pretty self-explanatory and along with a short comment, this is where you add the original FOR SALE thread ID.
If you have any more difficulty, send me a PM and I will try and talk you through it.
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