How do you get unlimited stuff at pokemarts in pokemon diamond?

To get unlimited items at Poké Marts in Pokémon Diamond, you'll need to utilize a glitch known as the "Item Duplication Glitch." First, go to a Poké Mart and buy an item that you have in your bag, then immediately after, open your bag and select the item you just bought. Next, while the item is highlighted, press the "Start" button to open the menu, then quickly exit the bag. When you return to the Poké Mart, the item will be duplicated, allowing you to stock up on unlimited quantities. However, be cautious, as this glitch may corrupt your game data.