Just to drift back onto the real world examples, there was a case where Britian sold advanced jet technology to the Soviets, which they then turned around and sold to the Chinese. The engines developed became the core powerplants of the early MiG jets.
Because of this, in the US, there was talk of serious sanctions against Britain. It didn't come to that, but it was a near thing.
Today, the US and UK completely share tank technology; the M1 Abrams, and British Challanger tanks are, technologically, nearly identical, yet, were Britian to suddenly turn around and sell the tech to Iran, they'd likely be barred to US tech for life.
Shortly before WWII broke out, Seversky Aviation sold an entire squandrons worth of their P-35 aircraft to imperial Japan. While it was technically legal, and we were not yet at war with them, the public backlash nearly destroyed the company, and force a major restructuring. (Think Aurther Anderson, here.)
Nobody likes to tell an informant anything, because anything they tell is going to be up for sale. If a country sells tech to another country's enemy, that country is going to be exceptionally wary about selling tech to the first country.
Basically, set it up, so if you sell tech to someone's enemy, and they find out about it, they will be less likely to sell you more tech, and less friendly with you. Selling tech between friends remains fine, but playing both sides gets more risky. Its still profitable, but risky.
Harry Voyager
