There's another forum I'm on that has a "SOLD" button for the seller to click, that automatically locks the thread and marks it SOLD....
Here; it would be hard for us to tell something should be locked, unless we opened every single ad and read it. Some guys never go back and note it in the ad, so those would get left.
There is also no EE rule directing members to mark threads as SOLD, so we would have a new one to add to the list, and a new one to enforce.
It's not a bad idea, but with the system as it is here, it would be a lot more work on our end trying to find out what was and wasn't sold, and informing people that they had to mark ads sold.