The history of the mighty mighty Pokemon Dollar!
Pokémon Dollars are acquired primarily as prize money from defeating Pokémon Trainers in battle, or by selling items at a Poké Mart. Pokémon Dollars can also be acquired by using the move Pay Day in battle, at the rate of either 2 or 5 times the level of the Pokémon using it, depending on the generation. Additionally, some other moves like Happy Hour, as well as held items like the Amulet Coin or Luck Incense, can also have an effect on the amount of money earned in a battle.
Pokémon Black and White introduced the concept of item maniacs. Item maniacs are NPCs who will pay large sums of money for certain items, most of which have no other use but to be sold to these people. However, Pokémon X and Y removed these characters, and instead, items previously sold to item maniacs can now be sold at any shop.