Although the quest parts tell you to 'buy' the decorations, you can simply place them from your Inventory, and they are counted. This means that you can do the quest with just 1 Hammock, 1 Red Flower Pot and 1 Red Marble, by repeatedly storing then placing them.
You should only need to buy 1 Hammock (for 425 Silver Coins) to complete this quest, as you will probably already have a Red Marble and Red Flower Pot in your town from previous quests.
