Setting up Glass production is slow, so it might be advantageous to buy the required Glass from Lord Northburgh and rush the construction of the Keep, especially against aggressive Opponents.
The Keep is unlocked at 510 Patricians, and requires 5 tons of Glass to build. The two most obvious examples of this include buying Tools at the start of the game, and buying Glass to build a Keep. Active buying should really only be used to import Goods that you cannot or are not producing yet. The main difference between Active and Passive trading is that Active trading is done at the ports and harbours of others.
