As a Pinside+ user that browses the market subforums frequently, I would like to be notified any time a new thread is started, so that I am less likely to miss out on a "for sale" thread for a hard to find part. I believe this would add great value to the paid Pinside+ features.
Notification for any thread posted globally is good enough. Being able to "subscribe" to individual subforums is even better. I use this a lot on stock Xenforo sites and really like it.
Example of why I'm requesting this: Recently a PFS thread was started with a couple of hard-to-find WPC transformers for sale. I missed out because I didn't know they were listed until I got home and checked the site.