You can't close a topic on your own. Once the topic is done, you can stop bumping it, and if possible, edit the first post to 'done,' 'closed,' 'sold,' or similar.
You can edit a post by clicking 'Edit' located in the top right corner of the post itself.
For regular posts on the forums, you are able to edit your posts for 30 minutes. This expands to 1 hour for site customers and 2 hours for verified users.
For the first post in a trade topic, you are able to edit the post for 3 days by default, 5 days as a site customer, and 7 days as a verified member.
Editing the first post of a trade topic will reset this timer.
More about customer and verified user perks can be read here:
https://forums.d2jsp.org/verify.php