Instant notifications on your phone with messages or responses to post.
...site does this already if you enable it.
Or even alerts when certain items are posted for sale using key words like on Craig's list
...site can alert of posts to user-specified sub-forums already. Yes, I understand you want notified based on keywords - that functionality doesn't exist but it can... and ya don't need an app for it. Request the enhancement. Maybe they'll oblige.
Hopefully that helps ease your pain.