A minimum signing in period of around every 3 months would be reasonable.
Say after 3 months someone has not signed in to their account ebid could send an automated email to the seller asking that they sign into their accounts within 31 days to keep the listings active and show that the seller is still serious about selling on here.
Anyone who wants to still sell on here would sign in. Easy. If they don't receive the request due to having an old email account on here then it's not a problem with removing the sellers listings as they would not receive any notifications if they had sold items or likewise (And i'm sure there's a few on here).
I had given another auction site a crack some time ago but with no luck. I recently received an automated email requesting that i sign in after 14 days or have my listings removed. I decided not to sign back in so i'm guessing my listings do not exist on there any more which sounds reasonable enough.
Now what's the criteria for joining this 'new' perverse group?