It is disabled.
From B&S Guidelines:
Just a note that the Edit/Delete thread/posts functions are disabled in the Buy & Sell forum.
This is to prevent misuse or abuse in the event that a buyer or seller edits the posts to "cover up" or otherwise post irrelevant things.
It will also serve as a records of sorts on particular buyer/seller behaviour in ClubSNAP.
If a member needs to change something, post a reply which supercedes the original information.