before you keep reading: this is only for trade forums!because otherwise it could be abused by stalkers or other weirdos
i think this would be good because when someone posted in your topic and went offline right after...
now you are online but the other user is not, and you get a notification once they are online again,
you could directly contact them and do your trade.
you know, we dont always check when other users are online while we are online. sometimes we are both online but don't realize it.
add some limit to this notification so it cannot be done with lots of users at once. and please if you
like this suggestion make it available to all users.. not only to those who pay for it every month or who got a star next to their name!