Only the website itself is copyrighted; user-made content is not.
There are no real rules against copying/imitating limited content created by another user. Unless it involves fraud, defamation, or outright plagiarism, the content probably shouldn't be administrated. So if your auction title gets copied, there's not much you can do about it, save politely asking the user to stop.
It's still not in one's best interest to copy other users, nor to become upset whenever someone seems to copy you. But it is to create original content, give credit where it's due, and just accept certain things.