Gold is something you have to have at all times if you have troops. Otherwise if you get to 0 gold, you will lose all your troops. It is better to max out on building lumber and the other resources than saving a little people to earn gold. Feel free to disagree but the math doesn't lie.
For example, if you can sell 10k lumber at 10 pieces per, you gain 100k gold. On the other end of the spectrum, you decide I don’t need that extra wood and rather have the gold, those people earning gold instead will earn you 30k gold instead (since one person at the lumber yard counts as 3) Which would you rather have? 100k gold or 30k gold? As long as you are selling the lumber for more than 3, you will gain rather than lose. The only con of it is, those transporters out trading could be used in pillaging. BUT the beauty of it, you can afford to get more transporters faster. The more transporters you have the less trips you have to take and less time waiting to do things. All depends in what you are doing and your playing style.
it goes by what people are buying. Always look and see what is selling highest. You'll want a decent trading post in all your cities then