I was playing a game where I believe I was Drengin. The Terrans decided to declare war on me. I had no military to speak of as I was building my civilization.
I went to the diplomatic screen and purchased the entire Terran fleet. I then used it to conquer the Terran planets the ships were already in the neighborhood. They didn't have enough transport ships so I upgraded the colony ships I purchased to transports. The first thing I did is take out their shipyards.
Now I like that I can converse with the other civilizations at any time. I didn't like the timeout imposed in GC2 where you could not speak to a civilization for a while after you had spoken to them. However If a civilization is currently at war with me, it probably shouldn't sell me its military equipment unless a peace accord is part of the deal.
Piracy:
If it was possible to use another civilization or pirates as intermediary to trade for tech / ships from civs we are at war with (black market). Giving a letter of marque to Pirates could also be a possibility so they could capture unarmed ships (or armed ships if they have the firepower to force a surrender) from civilizations we are at war with and they could sell them to us for less than they are worth. The pirates could use this method to gain strength every time there is a war. Using their money to acquire ships and/or tech they copied from the black market negociations. For example If I use the black market to purchase missle tech from the Krynn, then the Pirates would have this tech as well. They could upgrade the ships they acquire. A starbase that would be both Military and Economic could be their home base (with the pirates not having a home world.) The AI could also make use of the Pirates.