One about atmospheres

[Suicide Junkie]

It's possible that everyone else could trade colonization techs and population. Even with only one tech and one race trade, your benefit would dissolve

Having a rare tech and population means that yours are more valuable.

You don't have to worry too much about the other player reselling your tech, since he isn't going to give it away for free. There is only one other colonization tech that he could get in exchange for it anyways.