Go to www.soundonsound.com/readersads (use the grey button on the Home page's secondary menu) — then, over on the righthand sidebar (when viewing the site on a desktop computer) you will see three red buttons (these are lower down on a mobile screen so scroll to find them).
Click the middle MY ADS red button and it opens a window and gives you a link to delete any ads for gear you have sold. Note: you cannot edit your adverts, so it is best to copy the text first, then Delete the advert, and paste the text into a new advert.
You can also delete from within your MY ACCOUNT > MY ADS tab provided you are logged into the site.
SOS is unable to delete adverts for you — alternatively leave them published, because they auto-delete 30 days after they have been posted on our site.