I've occasionally traded with some townsfolk to short-circuit a quest with a poor drop rate after googling to figure out what mobs I needed to kill, but it seems like, for the most part, all any of them carry is useless junk.
Do any of the townsfolk ever carry anything useful or rare, such as Arts Manuals or better weapons/armor/gems than I've been able to acquire before meeting them?
If so, how can I tell? Is there a list anywhere? Or is trading mostly just a waste of time?