Yes, the editing time window is 4 days. After that period the Edit button disappears from your posts.
For any classified threads, simply add a post saying the item is sold. That will prevent any inquiries from the 95% of members who actually read the final post before posting themselves.
If it is imperative that a post edit be made after the editing time is over, as SilentHitz said, simply PM any moderator who will be able to perform the edit.