One of the most cheesy ways to exploit tech trading is as follows..
Totally ignore weapon/defense tech (just stop at space militarization which provides +10 military bonus), while working on peaceful techs.
When the Ai starts getting itchy, you start preparing a military for defense. Find some Ai opponent (minor races is fine) and trade for their top or second top weapon tech, thereby saving on all the research in between. You can leap frog from nothing to Harpoons III or better depending on the AI difficulty.
This loophole benefits the humans a lot more than the AI because typically, the AI has very balanced research priorities.
Also this tech leap frogging effect is strongest for the weapon/defense lines (which the computer pritorizes and is often ahead) because in 99% of the cases, there is no need to acquire lower techs items, unlike say in other tech lines where techs give bonuses. Heck in some cases, acquring a top line plantery improvement without acquring those in between can even hurt.....
Granted the AI could be tweaked to consider not only the RP cost of the tech, but also the RP tech in between saved, but that's not easy. I think it's easier to simply restrict tech trading so if you trade for a highly advanced tech that you don't have the last N (say 3) prequisite for it, it is simply too advanced and you can't understand it.
Say N=3
So you can't trade for laser IV unless you have at least laser I .
Maybe the AI can jeer at you when you ask for it, by saying "This tech is too advanced for your primative culture" or something.
Similarly tech stealing via spying or conquering planets would be restricted in the same way.
If you want to get fancy, you can allow such advanced techs to be bought/stolen but they don't kick in until you research the prequisite techs, and these prequisite techs get a research bonus...
Altough, the AI will never ever be smart about tech trading as humans, I think this is a fairly simple improvement that can be added in which will close off one of the ways humans exploit tech trading.