There is a limited time to edit a post. Maybe up to 2 hours. Haven't seen any documentation about it though. If you want to change the price, just tag on another new post to the end of the thread.
Members can not delete posts.
If a posted item has been sold, just add a post that the item is sold. It may catch my or other moderators' attention, then we lock the thread and move it to old/sold.
We can hide posts if requested, or edit to fix a problem, etc. Just report the post and describe the problem.