They lock threads to avoid spammers and also hijacking of threads. One good example is the for sale thread of TheButterZone like this one [WTS] Contents for a small thrift store for you or for charity and TheButterZone explained on the very first post why the thread is locked https://bitcointalk.org/index.php?topic=636950.msg7102804#msg7102804

There's Pros and Cons on this style of selling items in a forum, there is no problem if the seller is very well known, but if its someone like a newbie or one with no trade history its suspicious why transaction needs to be private.