If you write to the animals that live in your town, include a bag of 100 bells as a gift. The animals will always write back to you and include a gift in return - usually an item of furniture or a wall / carpet. This will not only increase your catalogue list, but the items they send are always worth more than 100 bells if you sell them back to Nook - so you are making money each time.
When you shake a fruit tree, instead of selling to Nook, plant a new tree - they grow in three days and you'll have more fruit to plant and sell. Pretty soon your town will have plenty of fruit trees which you can shake every 3 days to sell to Nook.