The reason is that people don't want their DMs to be overlooked (you never know how many messages and with what frequency the seller receives) so, they kinda ''insure'' their message, by posting ''pm sent'' on the wall, so the seller knows he got something from this person.
I can't think of any other reason. I personally do it very rarely, and only in such cases when I positive my name sent could be exactly what the seller is looking for.
Adding "noindex" meta tag to "pm sent" posts could be a good solution if "pm sent" posts are indispensable. Adding "noindex" tag could update all the existing and future "pm sent" posts very quickly and dynamically.
If "pm sent" posts are not indispensable, existing posts can be quickly removed with "find and replace" from mysql at shell as root or via phpmyadmin.
Please take backups of your database and files before editing files or database.