As Fred says, the edit button is only available for a short time, maybe 2 hours. I've never found out how long it is open for.
Just post an update to your thread that the item is sold. I follow the for sale items watching for the items to be marked sold, then I archive them. (Lock to prevent any further commenting and move it to the forum old/sold). I also check now and then for items that have been listed for over 2 months and archive them unless someone is actively involved in purchasing.
If you have something that needs editing and it's past the open time, report it, or send a moderator a message with info on what needs to be edited to correct any wrong info. Else just add a comment to the thread with any information you want to add.