show the date a seller has last logged in.

I can see the value to that.  I will have to think about how/when to present that info though to make sure we don't interfere with user's privacy.  I can see some buyers or sellers using that info inappropriately in their communications with each other.  "You logged in yesterday but you haven't shipped my item I bought yesterday.  When are you going to ship???"

One of my first market place purchase experiences was with a seller that has all positive feedback and dozens of items for sale. I ordered a game on 9/05/23 as of today it hasn't shipped. I've sent multiple messages and never got a response cause they have logged in. If I had known the last time they had logged in I would looked else ware. This is my perspective for this feature request but I can understand the problem you mentioned in your post.