The most important thing about buying items is haggling. Customers won't always offer you what an item is worth, and most of their offers are going to be high. The cheaper you can get the item for, the more profit you'll make, so haggle with them as much as you can.
To haggle, use the slider or the input box and offer the customer a price. If they accept, you'll buy the item for that price. If they don't accept, they'll lose patience. The lower your offer, the more patience they'll lose.