There should definitely be some osmosis for tech and civics between nearby Civs, but I think directly trading them is the wrong way to do it.
This. Tech Diffusion among Civs with contact should be normal in any game, but specific Tech Trading, as posted above, assumes that a new 'tech' is far too easily absorbed and adapted between potentially far too different cultures/civic/social policies, and existing Tech.
As a IRL Example, does anyone seriously think the Lakotah Souix could have traded for Iron Working, and then built the Mines, Bloomeries and Forges and Foundries required to actually make Iron Working work? Possibly, but not likely, and in any case not without considerable time between learning what to do and how to do it and being able to apply that - especially when virtually all the skills required are very different in a society that didn't have any kind of metal working 'technology' other than cold working of copper.
And that's just one example: the other contemporary example would be the Chinese, who sent people to schools in the United States, had one of them buy an entire machineshop, set it up in China and within a few years were building steam-powered gunboats as good as any being built in Britain.
BUT they started that process with a long history of metal/iron working and ship building and industrial organization.
Now try to include those two examples in a single Trade Tech mechanic with dozens of different Civs in different stages of Technology when they try to trade - and without forgetting City States, each with potentially their own technology level.
But Tech Diffusion has been normal since the beginning of History, and has accelerated since printing made spreading all information so much easier. As a prime example of how to implement this, note that the (Dreadnaught) Battleship as a technology spread to everyone who saw the HMS Dreadnaught launched - it was pretty obvious that it was a Big Ship with 10 or more Big Guns when all previous 'battleships' had no more than 4 Big Guns on a much smaller hull. BUT building 20,000 ton ships with big guns and high-powered steam machinery is not something you can do in any ol' machine shop - 10 years after the Dreadnaught launched, in 1916 there were still only 7 nations in the entire world that could build a dreadnaught Battleship: Great Britain, United States, Germany, France, Italy, Austria, Russia. Argentina, Turkey, Brazil, Chile, Japan all had to have their first battleships built for them in those countries - in 1917 Japan managed to build the hulls, but still had to buy the machinery and main guns from Britain and the USA.
And, I propose, that's how Tech Diffusion gets limited. You may 'get' the Technology from a neighbor or trading partner, but being able to Apply the technology would require, in some cases, a lot more effort and application of resources.
And if Civics/Social Policies are kept separate, they should also 'spread' to other Civs, with frequent consequences that are Not positive - like Religion, that new Social Policy might be hugely disruptive just when your Civ is not in any condition to easily handle Disruption. Too bad, think of it as a Policy Natural Disaster . . .
