i repeat: you do NOT need a trader building to trade. (And this time, i am sure; i just tested)\

Edit: let me reiterate that this INCLUDES THE FIRST LEVEL. Load up the julii claim and note that eturia has no trade building but around 200 in trade, including sea exports to carthage.

Each level of trader grants around a 10% increase in trade. Since exports are now capped at 1/level of port, this makes the trade buidlings absolutely terrible economic investments.

On the other hand, the first level grants an 0.5% bonus to pop, and all the trade facilities are necessary for some of the other upgrades (notably, the blacksmith line). SO you might want to build them for those reasons.

But if you're looking for income grwoth, look elsewhere. Trade can generate a nice bit of income, but building trade buildings is a waste.