Trading seems very inconsistent in general. Although I don't have any way to confirm this, I suspect that the AIs trade military technologies among themselves. However, they refuse to trade military technology to me under any circumstances - even if I have them bent over backwards over their last star system. Even the race known as 'mercenaries' willing to sell anything to the highest bidder absolutely refuse to sell me these technologies, no matter what the price. However, when I was manipulating the amount of credits just to see what would happen, the trade text turned 'green' very early on, signifying they would be willing to trade for that amount of money, but when I clicked ok, the text turned back to red with the disclaimer that they couldn't possibly trade military techs. |
I'll just say that I've found the AI more then willing to trade military technology with me, both offensive and defensive. This is on Challenging. Similar to Civ 4 I believe your relations play a big part. If you are on friendly terms or even warm the AI will give you a better deal. If they hate you then they won't give you such things without a high price.
One thing I'll do also is trade military techs for military techs. Hmmm, I see they are starting to build ships that defense against my beams, well, how about trade them beam tech for missle or mass driver tech. No problem says the AI, we like military techs. Sweeeet. Now I can build missle ships they have no defense for.