Yeah, like all of you I'm having the same problem with enemy tech trading. And yes, it is not realistic AT ALL! But like No.Dice said, buy the most advanced technology and resell it to all the other civs. I've been playing on Monarch, and it's an absolute life-saver, though I also hate playing this way.
I had a weak, backwards nation bowing and paying tribute to all who demanded it. Once, though, I had a new tech to trade, swapping for gold or another tech as necessary. In a series of negociations (21 in fact) I acquired all the world's current technologies and was earning +200 per turn even with science at 100%.
Two hundred years later (1340's AD) we're all entering the Modern Age and I'm gaining an average of +1000 per turn. Pretty good and useful for a small fifteen city civilisation (the smallest in my world)!
I'm still weak (at the moment), but I tell you, I'm real rich! So I guess the moral of my little story is if you can't beat them, join them and milk them for all they're worth!
Also, I reckon that it's useful if you're the sort that likes playing during the modern age, because everyone is researching at the same level and advancement occurs more quickly. Until all the techs are researched and sold, you're rich, and then you've got about FIVE HUNDRED years to build and unleash the ultimate modern army. Yay!!!
:soldier: :tank: :soldier: :tank: :soldier: