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.
- Poison Crystal - Requires a King Insect's Shell, Toothpicks, and Honey. It's a Crystal that adds poison to your attacks.
- Blazing Charm - Requires Gunpowder and a Water Mushroom. It gives magic defense like an Amulet, but it also reduces Fire damage even further.
- Amber Shield - Requires a King Insect's Shell and a Slime Liver. It's similar to the Blazing Charm, except its a defense shield and it applies to Earth damage.
- Dragon Buster - Requires a King Insect's Shell, a Wyvern Horn, and a Broken Seal. It's a 70 ATK gauntlet item with a fancy name!
- Ancient Gramaphone - Requires the Gold Horn. It unlocks the section of the Jukebox Mode pertaining to the last level and the ending.
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.
- King Insect's Shell - may drop from the King Insects in the "Insect King's Nest" stage. Automatically gotten on kill, you don't need to pick it up.
- Honey - may drop from a Beehive. Automatically gotten on kill, you don't need to pick it up.
- Wyvern Horn - may drop from the Aeolian Wyvern when destroying a horn. Automatically gotten on break, you don't need to pick it up. You'll be making a lot of attempts to get this one.
- Gold Horn - guaranteed drop from the Golden Wyvern when destroying a horn. Automatically gotten on break, you don't need to pick it up.
- Toothpicks - may drop from Hedgehogs
- Broken Seal - may drop from Ghosts, large or otherwise
- Slime Liver - may drop from Slimes, large or otherwise
- Gunpowder - may drop from Bombs
- Water Mushroom - may drop from Mushrooms