From Bulbapedia, the community-driven Pokémon encyclopedia.
The Croagunk Swap Shop is a shop located in sublevel two of the Wigglytuff Guild, found in Pokémon Mystery Dungeon 2. It is run by Croagunk. Here, Pokémon can trade various rare items received from Treasure Boxes for other items. This shop can only be accessed shortly after the player finishes their initial visit to Apple Woods.
For a list of all items that can be traded in the Croagunk Swap Shop, see trade items.
You can only trade items if you own the entire set of what you are trading