I believe you're referring to a post in the Wanted/For Sale Forum?
After posting a message, you have I believe two or three days to make edits. Moderators have a longer period. After that period, if you need to make edits/changes to a post which cannot be effectively done by adding another post to the same thread (say you wish to add the words "SOLD" to your title so people will quit asking you about it), then you have to enlist the assistance of a Moderator or the Admin (me).
Christian's way of doing this is correct, but an even easier way to do this is to click on the little "Report" button at the bottom of each post. This will provide you with a window to put in what you want done, and a direct link to the post for us to follow, as well as notifying all of us of the presence of your request. You can also use that link to report objectionable posts or other things you find that you have issue with/or are a problem.
These time limits were instituted a few years ago to protect the database from wholesale changes to threads, in order to ensure their continuity and completeness, and were added to the
User Agreement during that period as well.
Please feel free to let me know if you have any questions or concerns about this policy.
Thanks,
//sse