A lot of folks are not courteous enough to actually go back and change the listing flag to "SOLD" once they have the money. The usefulness of the listing is done for them so "meh". If we're lucky, they may come back and bury a "sold" posting in the thread. They simply can't be bothered to figure out how to go back and edit the original post and flick the little flag.
I personally wish that after a set period, say 3 months, the listing would expire and go away. Or at least the flag would change to "expired". That way you don't get folks resurrecting 6 1/2 year old listings going "WHAT A FIND! Is this NIB MK10t still for sale at $200?" It puts the onus on the lister to not waste other folks time, and if they still have it and intend to sell it then they need to renew the listing.