I believe to prevent confusion we should post at the end of a thread tht its done so their is no confusion for new people. Specifically the for sale threads because honestly even if the thread is multiple years old unless someone says hey the ships are sold them it will appear that they are not sold I have no doubt that it will be a long time before mine gets sold if ever thats just how it is. this would prevent confusion for new people who may really want to purchase a ship second hand to get started. Just my thoghts.
We do ask that people edit the title of their For Sale threads with [SOLD] so that this is clearly established. But I can't hold a gun to anyone's head and force them to. And I'm a little too busy to go back 2 years and close out all the old threads that have recently been necro'd, so there will probably be some threads hanging out there. But I'm in agreement with your fundamental point.
In the line at the top of the first message (in the white part where on the left it has the date and time), to the right side should be some buttons for 'Delete', 'Edit', 'Quote', etc... click on edit, and you can add [SOLD] or [FOR THE LOVE OF GOD DO NOT NECRO THIS] (I like the second choice). You should only be able to edit or delete your own threads (unless you're mod on a subforum), but quote and reply should be active.
I've been deleting necro posts when they add nothing to the thread, and in some cases locking necro'd threads to prevent further pointlessness, but in general yes, people should be marking their threads [SOLD] or [NO LONGER NEED] or [FTLOGDNNT!] and it would be awesome if they did so.