A few things Ive noticed when trading.

You don't seem to be able to receive diplomatic capital when you trade for it at all.

Can we get techs that you cant use (ie opposite fraction only) ones removed from the list of what you can trade for and with?

Perhaps the value of a tech you are trading for should be the sum value of all the techs that lead up to it.

If I have a resource, and my trading partner doesn't have access to it at all, shouldn't it be worth more, and via verse? I would be thinking about 5 times the amount.

 It would also follow that if your trading partner has more then twice(and also at ten times) the resource then you, its perceived value would be less or worthless.