There is no good way to implement the kind of "tech for tech/cash" trading seen in previous civs. There is no cost to giving someone your tech unless you don't want them to have it; however, if someone else is going to sell it to them in the near future then even this cost becomes irrelevant. Therefore the rational market price of techs will always tend towards zero after more than a couple of civs get the tech. This means that, if AI is rational, tech research only matters for the top 2-3 techers, but has no relevance to anyone else, who can just buy all but the most cutting edge tech for a single gold piece. On the other hand, if AI is irrational, tech trading is just an arbitrary advantage to the player.
Top level play shouldn't be all about exploiting AI stupidity.