Create a red/amber/green indicator dot next to the merchant's name, where
  • green: The merchant has logged in at least once in the past 90 days
  • amber: The merchant has logged in at least once in the past 360 days
  • red: The merchant has not logged in in over a year
This is the easeiest way to find out wether any store on the MP has a halfway active owner. It will come with the least possible overhead (no counting how many logins, no tracking how long the logins are), but will still give MP customers a basic indication how active a store is. Also the suggested timeframes won't place too much of a burden on stores run by an alt.
Yes, it's not a perfect solution. Yes, some merchants might fall through the cracks. Yes, there is a lot of room for improvement and sophistication. BUT it is the most simple way todo it, AND will be helpful for customers.
Note to the Linden checking this post: I was an avid reader/contributor to JIRA, and Canny since the switch - up to my knowledge, this super-simple indicator was NOT brought up before.