Adding [SOLD] at the beginning of the title of the thread once an item is sold definitely makes it clear for members who navigate the Classified section and look for an item still for sale, or want to see how it went for an item that has sold.
It's done, although it won't reflect all the way through the discussion. Only new posts like this one.
I would add it at the beginning of the title because when there are many threads, it will be much easier to read and navigate the Classified section.
ex:
[SOLD] Corbin Seat
Brake levers for sale
[SOLD] Used RLETs for sale, asking for $200 each only
[SOLD] Heated gloves