Bots/trolls don’t care about having cool handles so nothing from them would be valuable enough to pay for. As you said, it is now about the inactive handles that were left, for example npr. He hopes that maybe this will make them come back or at least he can make some money if real trolls will purchase them. I think he was waiting for a year, because the former is worth more. The later generates some income, but it dilutes the value of Twitter.