The basic inventory can become full rather quickly. By upgrading it you can stock more items and therefore earn more cents selling Items to Sader. To upgrade it, open it by tapping the crate icon at the top on the screen. A list of items will be shown in the bottom left corner that you need obtain via animations or crafting. Alternatively you can use bills to purchase the required items. If your inventory is already full, you will need to tap the X button on less desired items to remove some in order to obtain the required items to upgrade the inventory..
1. Tap the wooden crate icon at the top of the screen
2. When you've obtained the required items, tap the "Upgrade" button to confirm the upgrade.