There is no way to delete a thread yourself. You can PM a moderator and ask that a thread be removed but if they got contacted by everybody for things like that it would be a full time job. Most people either just edit their original post to whatever they would prefer it say, or, if they don't want the ad any more at all, they just delete the contents of the first post and list the reason such as "item no longer for sale" or "item has been sold" or whatever the case. In the case of a sold item many people leave the ad up but just replace the asking or selling price with "item is now sold" as a courtesy to the new buyer so that the price they bought it at isn't public knowledge if they choose to sell it down the road.