Or, they could have implemented the Diablo 3 trade system, where you could only trade with people who were in your group when the item dropped, for (IIRC) up to an hour after the group disbanded.
It’s been ages since I’ve played D2, but I remember being able to trade with people there, too. Maybe I’m misremembering, I don’t know, but I’m pretty sure trading has been in the Diablo franchise, in one form or another, for a long time.