By year 2 in Stardew Valley, your farm should have more than enough artisan equipment to increase the quality of your crops. While it's obvious that some fruit/vegetables sell better than others, you can potentially enhance their value by aging them in the farmhouse's cellar. One type of fruit that you'll want to grow and age in bundles is Starfruit. It sells for a hefty price that can be raised depending on the Profession you choose when leveling up the Farming skill.