Right you can only visit one Tradepost per turn.
As long, as the towns in the hanseatic league are small the TP's in the med are small, too.
There are always 2 goods they buy and 2 they sell.
They will buy only between 40 and 100 barrels as long as they are 0-2 star TP's. Depending on the time of the last visit.
But they will sell between 100 and 200 barrel of each of the two goods they offer.
Your experiance, that you only got 40 barrels of spices must have two reasons.
First a AI trade had visited that town shortly before your visit and second, your convoy was filled to the last hold with pitch. You sold about 100 barrels. Second, the captain didn't fill up to the last barrel. If you had send only 200 instead of 800 barrels of pitch, he might have sold 100 pitch and buyed about 100 spices and 80 barrels of the other good (cloth or wine).
Hein
There is nothing more horrible than a won battle, exept a lost one.