Golden Goods

Run by Millionaire, you can buy and sell weapons, accessories and items. Also he will give you the option to buy a bigger knapsack, which will allow you to hold more of each item. They come at a price though.
Basic Knapsack – Holds 12 of each item – 2500L
Large Knapsack – Holds 18 of each item – 10000L
Jumbo Knapsack – Holds 24 of each item – 30000L

Note also, items sell back for a tenth of their price, so don’t expect to make a huge amount from items found on your first quests.

Bomb Bros R&D

Run by the Bomb Brothers, you can buy and sell gems here, and fuse them. For more details on the properties of gems, their types and gem fusion, see the Gems section.

Note that gems you sell will obtain a 5th of the price you’d pay to buy them, which is slightly farer than Millionaire’s deal (which is why he’s a millionaire, I guess).

Dud Services

Run by the Dudbears, they collect job requests from all over the world and post them here to find help. You can only take one quest at a time, and have to pay a service fee when you take it. When the request is completed, come back to the store to receive the reward.

The rewards will increase in value as you complete main quests in the game. Also, from what I have observed, you won’t be provided with a new set of 6 jobs until you complete either one of the current jobs, or a main mission in the game.