I have come to a situation where I could sell for more than I could buy.
The trader was selling coal for 5 and buying it for 7!
This meant that I could buy everything and sell back for bigger price and then buy everything back. With each transaction a was earning 2 gold per inventory (it was coal in this case).
Developers, please take notice of this bug, don't allow traders to sell products for less than they are willing to buy.