From Bulbapedia, the community-driven Pokémon encyclopedia.
An item maniac is a person who will purchase certain items from the player at a higher price than what they would be sold for to a Poké Mart. Some items which cannot be sold to Poké Marts can be sold to item maniacs. They appeared in Generation V.
A Maid in a trailer on Route 5 will sniff out any gourmet cooking ingredients in the player's bag and offer money for them. She gives the following amounts:
The prices she offers for drinks are the same as what the player pays at Vending Machines. All offers are higher than the prices offered at a Poké Mart.
A Gentleman inside the Icirrus City Pokémon Center will offer to buy a variety of stones, gems, and other items from the player.
A man from the Riches's house, who is dressed as a Black Belt, appears in Undella Town. He will purchase Relic and flute items from the player.
Rare Bone maniac
A shady man in a house on Route 18 will buy Rare Bones from the player.
A Pokémon Breeder at the end of Mistralton City's runway will buy Mulch from the player.