The Experience for Monsters in MapleStory
Eventually the exchange becomes merely a way to clear useless etc items for usable items in Maplestory, though selling them for pure meso is always an option. Solid horns are usually preferred to level up a lower-level character, due to the number of players training on the Jr. Kitties at any one time. Zombie Teeth are more expensive, but the chance of getting a Fairy Wing as a reward can offset this extra cost.
Also, players train on Zombies far longer than Jr. Kitties. Jr. Yeti Skins fall in between, more expensive from a higher-level (Maple Story mesos) monster, and providing better potions than Solid Horns, but nothing too special. Every exchangeable etc item has a specific scroll type (usually 10%) that has a small chance of being given as a reward for a trade-in.