Recently someone fished up a bat dungeon key. Since Dungeons don't despawn until they're completed, and if the dungeon boss is behind the locked door then this key is absolutely necessary to be able to finish the dungeon, then eventually someone will need this key.
I'm imagining the not-so-distant future where these forlorn keys become plentiful, along with many known dungeons that can't be completed.
Why not have a bit of text in the tooltip saying the name of the dungeon that the key belongs to?
This way people can specifically say the key they're looking for. "Buying Batty McBatface's Grotto Key!" And similarly people could see in barter stands this very important info on the key.
Heck, maybe make the keys trackable if you've visited their initial altar?
Also a quick plug for my soil global pool idea from a while ago, these would definitely fall into that category as well. Not an item that should easily be lost forever. http://www.havenandhearth.com/forum/viewtopic.php?f=48&t=60468