Yep - do that, as well as click on the dropdown at the top right of the post, labeled Thread Tools, make sure that Close Thread is selected, then click Perform Action.

You have to do both, or the folks here that don't pay much attention to things won't see "Sold", and keep on trying to buy it, ask questions, etc.

Note to forum owner: A "Mark As Sold" icon available to users would be a HUGE asset here. Then the ad gets properly closed and filed away with one mouse click.