Instead of banning tech trading all together, why not make this an option?
The player(human or AI) can only sell techs that they have researched themselves.
Or in other words, you can't sell a tech that you have either bought or stolen from another race.
All that would need to happen would be to give technologies an extra binary variable.
A tech is set to 0 when it either:
- has not been researched
- has been bought from another race
- has been stolen through espionage/planet invasion
It is only set to 1 when it has been properly researched.
And only those techs set to 1 can appear in the diplomacy window to trade with.
The effect of this, is that you can no longer make a profit by buying a tech from another race and then selling it off to the rest of them. Also, in a sense tech trading will become more like research trading, which is what it should be.
This is probably the easiest way to negate triangular arbitrage, while still allowing some trading to occur!