He will do this until you have 59 total slots in your bag. In exchange for money, he will give you an extra slot of the bag for you to store items within. The bag upgrades are done by Bagin in the Galaxy Building.
You can rotate the items in your bag with a chest that can be found at base camp and your lodgings, but you may wish to upgrade the size of your bag. While you can store hundreds of each individual item, you can only have a limited amount of kinds of items, specifically 20 at the start, and you get an extra slot just before facing Dialga & Palkia. Unlike most past Pokémon games, your bag in Pokémon Legends Arceus only allows for a limited amount of items. New information on the Spirits Shop was compiled in a Reddit post by u/ZXLucario. This is where you can buy items to level your Spirits up, trade them in for cores to make new Spirits and a lot more. New game leaks from Super Smash Bros Ultimate also show that there will be a Spirits Shop.
