Trading is a black art which you can only learn by experience.
and by reading this forum :-D
It changes with different revisions of SEIV. I have found it hard to trade for Ship Construction Tech until later in the game. The AIs seem to value it more. Trading for Colony Tech was hard, then easy, now harder to get the AIs to bite. But it remains one of the best trades to make with them. I always try to trade with the AI fairly, but there are exploits that can be used. An example is trading three levels of Research and three levels of Cargo when you only have one of each. If you are playing solo and cheat, you only cheat yourself.
I think planets may be valued at the stuff they contain. I once traded a single planet for three newly colonised ones of his. They took it for some reason. Soon after, two of those planets were destroyed, so maybe the AI got the last laugh after all.
Many (all?) of the TDM races have trading turned off. They will not do a trade even if the best terms are given.