I believe the original poster (OP) should be able to change his or her thread title later on...
This is extremely important to reducing spam and getting focused attention in the trade markets...when you have a new item you can just change up old thread title instead of make new thread...

Right now we have to make new thread or ask Mod to change it for us...wouldn't it be better if we ourselves could do it? Of course to prevent abuse, the change thread title option could be limited to only threads in the Trade Market and allowed only once per day or something like that....