Are you playing on a diff level below Tough? I haven't done that in a long while, and it's possible that the handicaps for the AIs include limiting their trading activities. I've seen regular offers for techs and less regular ones for trade goods. But now that you make me think about it, the only 'offer' I recall ever getting for an entire colony was in 'give us that as a present' mode.

On the 'growing too strong' message, I've always just taken that to mean "We're getting scared of them, you look kinda tough and not hostile to us, why don't you do something 'cause we can't."

I always had the impression that the AIs are programmed to exchange techs between one another but far less with the player, perhaps in the spirit that it's up to you to suggest such deals? Though indeed they could become even tougher to play against if they actively tried to buy certain technologies or planets from the player.

With regards to the "growing too strong" thing, I have the same understanding as GW Swicord. It's a bit similar to when an AI is under attack and asking for help; they won't tell you what to give to them, they just ask for any help whatsoever.