When trading you can get much more money by having long term paiments (for less BCs per month).
I noticed that for me, if i select 1 I have two actual paiements (1 immediate, and one the next month). But I haven't checked if it is the same for the AI. I'd think so.
For selling you just select the tech or ship you want and then what you want for it (money or techs).
About the long paiements thing, if the race you traded with disappears before finishing paying you, you won't get all the money.
To buy ships from the AI you must have good relations
~SDC~
Stardock Forums