In Grace's accepted answer here it's mentioned that selling certain Treasure items will unlock new items in both shops. Since I want to make sure that I have the best equipment available before I try to complete both the game's story and the survival dungeon, I'm curious: which Treasure items can I sell to unlock more stuff to buy, and how do I obtain them?
There is a small handful of items that you can only acquire if you sell the right Treasure. These are sold in the small shop at the Familiar Forest. You only need to sell them to either shop, and it'll be available. I'm not sure about the need to sell more than one version of any particular item. If the name is in quotation marks, I haven't quite gotten it in the English version so I don't know the English name.
All of these treasure are drops from monsters. Unless stated otherwise, it will drop like any other kind of treasure and you need to pick it up.