Hey, thank you for your feedback!
Using instant buy/sell costs you money — it’s the price of convenience. If you sell resources passively, when the merchant visits your city, you earn a small profit. But if you travel and sell the resources yourself (e.g., by boat to another city), you make a much higher profit.
How much you earn depends on the method you choose — faster trades mean lower gains, while slower, active trade gives higher returns.
That said, trade and taxes alone aren’t enough to sustain your economy. War also plays a role, but probably needs a little bit of tweak.
Additional income sources are planned and will be introduced in a future update later this year.