On an older "For Sale" post, click on the seller's user name in the original post. Choose to "View Profile" - There it shows the last date and time when the member was logged in to the forum.
If the member hasn't been on the forum in a while, they may not see your reply post. In the Profile, you can also check to see if the seller has a phone number or email address that you can use to contact with a better chance for success. A "PM" or Private Message may also work if the member is inactive, if they enabled the option of email notifications of a PM message.