"you don't have to buy and sell at the same time during your action "
what does it mean exactly ? I'm not sure to understand. An example ?
You can sell or buy or sell+buy. AND/OR like the rules says.
Simple. That means optional actions.
Case 1:I sell 2 stone
Case 2:I buy 4 wood
Case 3:I sell 2 stone and I buy 4 wood