At the moment, there are automated feedback reminders sent out if feedback not received or given or something after a set time limit.
I would think it was a fairly straightforward thing to set up.
If it is that easy, couldnt an automated email be sent to any sellers who have not logged in for a set period.....eg 8 weeks.
If the seller then doesnt sign in for another set period, a second email could be sent out a few weeks later.
If the seller still does not sign in - then a message automatically be added to their listings - requesting possible buyers to contact seller before bidding or purchasing. This message would be removed when (or if) the seller signs in)