In order to help keep the section in order, I've enabled a new SOLD prefix to be used on the thread title.
Please use this when you have sold the item in question.
In order to do this, I've enabled a specific permission on this section.
Regardless of when the thread was started, you should now be able to see a Thread Tools link at the top of the first post:
Click that to expand it.
Click Edit Title
Change the prefix to the Sold one where applicable.
That will update it: