There's a man in Orbis named Staff Sergeant Charlie who is accepting etc drops from specific Ossyria monsters for 500 experience and will give you some rewards. A list of the tradable items and possible rewards is already compiled on Hidden-Street here. Some items are more efficient when comparing the gained exp to the exp gained while hunting the monsters. Specifically, the Solid Horns from the Jr. Kitties have the best ratio, as the only other monsters weaker in Ossyria are not accepted in the exchange quest of Maplestory.
Other items, like Bain Collars and Lycanthrope Toenails, are merely a novelty exchange, as the reward likely pales in comparison to the items gained while hunting down the monsters. Some items(Maple Story Mesos) are also bought in large quantities in use of leveling lower level characters. Though a character leveled by this method needs to be a minimum level 10 to reach Orbis, training is incredibly fast, gaining 500 exp in a matter of seconds. At higher levels, however, this method becomes less efficient.
Eventually the exchange becomes merely a way to clear useless etc items for usable items, though selling them for pure meso is always an option. Solid horns are usually preferred to level up a lower-level(Maplestory powerleveling) 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.