From Bulbapedia, the community-driven Pokémon encyclopedia.
An overview of various stands at the Swap Meet
A Pokémon Swap Meet (Japanese: ポケモンこうかんかい Pokémon Swap Meet) was featured in the anime episode Tricks of the Trade.
Ash and his friends were visiting Palmpona at the time of the annual event, which was a part of a broader week-long festival. It allows Pokémon Trainers to meet and trade Pokémon they may no longer want. Hopefuls groom their Pokémon and set up makeshift stalls in the hope of attracting interest. Great communication, bartering skills, and a sales pitch are required to entice another Trainer to trade their Pokémon. All trades are officially processed using a machine with a referee to watch out for any deception.
Pokémon seen at the Swap Meet