Unlimited money:
Purchase and collect items until a second page is displayed in the inventory. Remove all weapons and fill the second inventory page. Begin selling items starting at the last position on the second page. At some point the game will allow the same item to be sold multiple times. Continue selling this item to build your total funds.

Level select:
Press Right, X, Left, Y, A + Start at the starting menu to play a level that already has been completed.

All items:
Press Y, A, Down, X, B to get all the items at the shop.