Members can edit a post or reply for 10 hours after they posted it. Login and you will see an "Edit" button on the lower right of the box that holds your message. If there is no Edit button, the time limit has passed. After that, make updates as a another reply to that topic. Clicking the Reply button within your original post will thread your update to the original.
To mark your item as sold, post a reply within that topic to say the item is sold.
It is recommended that members use the "Preview" button before submitting a message in order to proofread. And to read it over again after it is posted so that edits may be made during the first 10 hours.
A Delete button may be available in some situations on the Edit page for a short time. Deleting the topic after the initial period is not available. Other members use the past topic pictures and prices as an educational reference to see what has been available for what price and for the enjoyment of looking at petroliana pictures.
If there is a serious need for an edit or delete, members may state the special case to me or a moderator by PM message or email.
When editing was more loose, we used to have a few people who would change the terms of the sale after a deal was made or after a dispute came up. Conflicts would then erupt. We also had a couple trouble makers who went back and edited posts over a long period of time to deliberately upset people.