Okay, you can now edit the thread title in your posts in Buy/Sell. You cannot change the content of your post/thread but you can put (SOLD) in the thread title once an item is sold.
Instructions for editing title in Buy and Sell forum:
1. Find your thread on the forum display page where all the threads are listed.
Double click the title bar with your thread title.
2. Your title has been highlighted and is now in an editable box.
3. Type SOLD after your title
4. Your title has now been edited and marked as SOLD.