SOLD
Unfortunately the title time-out can't be set on a per-forum basis, it's a board-wide setting.
To help notify people an item is sold in a classifieds thread you could always edit the original message and insert SOLD in large letters, as I've demonstrated here.
Alternatively, the original poster could report the thread and in the reason simply say "Item/s now sold, please edit thread title to reflect this." - that way me and the moderators will see it, and should be able to action it fairly quickly.
Cheers,
Shaun