1. STOP making plane deliveries. You lose money by doing so. You can resume once you've saved enough.
2. Sell lower. I have seen stuff stay for days in neighbours'side shops because they sell at the maximum price. Where as i have trouble completing my twelve slots because people buy my stuff sold at either market price or lower price. I earn 40 to 50k a day. Put yourself in the shoes of buyers. When you see two products at the same time which this game loves to do, you'd buy the cheaper one right?
3. Bestsellers are bread, cheese, flour, cream, feeds, dark choco, fish, flax and sugar cane
P.S. stew is so commonly sold that it is an item that stays for hours in my shop. I hate selling it.