Accessory ads are free and run for 60 days. After 60 days the ad is closed, cannot be purchased and will eventually be deleted by the system. Make sure you re-list your ad every 60 days.
Re-listing is easy, just go to your item and hit RE-LIST in the upper nav bar. All the previous info is in there, with a couple of exceptions. Complete those and you are good for another 60 days.
If you want MORE than 60 days. Let us know.