I wish they would implement a world unit trading market like I suggested somewhere else. You could then select units from nations that have earmarked them for sale, and then it would take you to a diplomatic offer where you could sort price out. Also you could have it on this so you could buy production slots, aka they would build x amount of a particular unit for you and this would be handled via the diplomatic offer screen. Keep unit blueprint buying the same but perhaps make it more expensive or just a license for x amount of units from that.
That's a pretty cool idea. You get the licence for P-47s, but you can only build 10. Want more, pay more (sort of like missile building in SR). An indef licence costs 10x what a licence for 10 does (example: 10 P47s cost in $$ is 1/10th the cost of actually building the airplane (if it cost $500,000 for 1 P47, a licence for 1 would cost $50k; a licence for 10 would cost $500,000, an unlimited licence would run you $5M).
I'd add that countries should only sell outdated licences unless you have alliances and top level relations. I can't think of a single historical example where a country in the CW sold a brand new weapon system to an adversary. Theft of plans, yeah, but no legal sales.