Here are some easy ways to make money.
Sugar: Sugar cane grows unbelievably fast, and is great for a quick buck.
Cobble: It takes a bit longer, but if you have a cobblestone generator, it's an easy way to make money.
Carrots: They grow quickly, and you can eat them.
Gold Apples: If you have a lot of gold lying around, and a few apples, make golden apples and sell them for $300.
If you happen to have a mooshroom, you can sell mushroom stew. I don't know if it takes the bowl, but it's a bit of money.
Lumber: If you have a productive tree farm. Sell the logs for a fair chunk of cash.
Mossy Cobble: If you find a mob spawner with mossy cobble, make sure you take all of it. It's worth a pretty penny in the shop.
Iron: While being the most useful item in the game, if you have more iron than needed, you can sell it for $15 an ingot. Not a bad deal.
Emeralds: These are rather rare, but they are worth it. At $125, they are worth 25% more than a diamond.