If you are unable to reply to a message directly, see if you can go to the user's feedback or profile page, and use the contact link there instead.
In some cases you may have difficulty sending messages if the original message was related to an item that ended more than sixty days ago or if the item was removed from the site -- if that is the case, going through the feedback or profile page should work. If you are prompted for an item number, either use a current number or choose "not about an item".
