Better (and easier) just to edit the post title to add *DONE* at the end, then edit the message body to reflect that items are no longer for sale. It can be archived then.
If you look at your thread there are a couple of icons in the top right of your post. Hover your mouse over them and a box will pop up telling you what each icon does, basically the little "pencil" one allows you to edit. The black cross allows you to delete, subject (I think) to there being no replies.
I'm pretty sure that if there are replies already then editing is your only option.
If you need a whole thread (plus replies) deleting you need a global moderator / admin to do it for you, as far as I know "Boab" holds those privileges as it's his site.