Well, the quest you need it for is the Patient in the Desert quest. If you don't give the lady all the items, you just get a different reward from the sick dude. All of the items he gives you can be bought at some time. If you get all of the items, which i won't, he gives you a golden amulet, that doubles your LP. This can be bought from a travelling merchant in the Rabanastre Lowtown, i'm just not sure when. I guess it isn't that big of a deal that i lost that loot item, since he will still give the the Barheim key that leads to a secret esper. I'm not gonna worry about selling loot anymore, there always seems to be some workaround.