Edit time is set to 60 minutes so for the first 60 minutes you can edit your post. This was changed due to some issues we had with editing. If the edit is important and can't explained with a secondary post a mod can edit it for you but for the most part we recommend simply explaining with a new post.
On sales threads where several items are for sale it's already common practice to make a new post with what items are left after a few have been sold etc. so I feel people are looking out for what's down the thread.