I think Xenocide is referring to the fact that when you trade with the AI, it always occurs on your turn, so that you can immediately turn around and resell the same technology before the AI can. Of course, I don't see why this is an advantage at all, given that the AI already had his chance (as he had the technology before you), and declined to do this (or already did).
The AI displays an extremely curious reluctance to sell ANY weapons technology, however. It baffles me as to how the AI races of the game can come to an almost uniform decision of what should be the dominating weapon paradigm in the galaxy given that they refuse to share any weapons technology, even horribly obsolete ones, for any price. I mean, unless you've been bootstrapped into committing to one type of weapon, there's no specific reason why you would choose the same weapon as anyone else. Indeed, if anything, I always choose an entirely different weapon if I am late to the weapons race (and I usually prefer to take a wait-and-see attitude), so as to have a weapon that no one else has a useful defense against, with the result that after the galaxy has already decided what the dominant weapon/defense paradigm is, I violate it like a schoolgirl in a Japanese cartoon.