The techs with a star are unique to that race in the sense that they are the only one able to research it. Once you acquire it through trading, you get the usual benefits from that technology.
Especially the minors are perfect to sell techs to early on in the game, as that can help your economy to stay afloat. The added benefits are that it causes the minors to develop faster; in particular technologies that give them access to tech / econ / manufacturing capitals are interesting if you're planning on taking over their planet later in the game
With regards to the other AIs you need to be a little more careful that you don't give them too much of an advantage. In any case (minors or other AIs) never sell them techs that grant a bonus to diplomacy since that will make future negotiations less fruitful.
The whole trade thing is a balance but generally speaking the AI tends to value quite a few technologies rather high, which leads to the "2 or 3 to 1" scenario you encountered. Don't forget you can also add up to 999 influence points to help make a deal. The whole balance strongly depends on your versus their diplomatic ability (see also the comment in the middle of the screen when youre trying to make a deal - this will indicate who has the diplomatic advantage)
After one year, you can easily get economic and research treaties from the minors in return for a few more advanced technologies. Some that I love are population growth techs and less advanced weapon techs, especially if the minors already researched another weapon branch there's little reason not to give them access to some basic stuff they won't use anyway but pay good money for.