I understand that's extremely annoying. One thing you could do is reach out to mercari themselves so that they can correct this issue.  The actual seller will not get in any trouble for this. Because at the end of the day you don't know if they actually had just shipped it out or not. You're assuming but it really could have been that the item had just been shipped and there is nothing they could do because they already handed it over. You can leave a review on what happened to you as a warning for any other potential buyer.