In the Shopping Mall, you must use CP to buy items from GreatMerchant or CostumerChen. You can check your CPs in the inventory.
Hovering over the item in the shop window, you can check its decription and know how much CP you need to pay for it.

How to get CP:
If you have Dragonballs, you may exchange your DragonBalls for CPs with CPAdmin (178,182) in Market. He will pay you 215 CPs for one DragonBall, and 2150 CPs for one DBScroll.

Buy Items::
There are two ways to buy items:
A. Right click on the item you want to buy, if you have enough CPs, then the item will come to your inventory automatically.
B. Drag the item you want to buy to your inventory. If you have enough CPs, the item will be in your inventory. If you do not have enough CPs for that, the buying is failed.

Sell items:
Drag the item you want to sell to the goods window of GreatMerchant or CostumerChen.
Note: You will get silvers, not CPs by selling items to NPC.