1st of all this, unlike any other game i'v seen up here, already is a
team game. So everyone already has 3 civ's to do relatively anything they want within.
What i'm am concerned about is players/opponents swapping/gifting cities anytime they feel like it or a player that is losing or wanting to quit that just gives away all but 1 city prior to leaving/quitting.

Both situations fly in the face of reality (yeah, i know, someone can come up with some
exception in some far distant history), countries/leaders do NOT simply give away cities unless forced to by war, or overwhelming cultural influence (which in the wisdom of Civ creators CAN happen).
As for banning
tech trading, i'v always viewed it as preventing the same situ's.
The quickest way to drive away players, especially new ones, is for experienced players &/or friends to use these sorts of tricks. I have no interest in running or even participating in that sort of "competition".

Taken to an extreme, U end up with a situation like the infamous GotM on this site. A small group of players & admin'rs that use totally unrealistic, stretch-the-rules ploys to run up ridiculous scores to "win". Hence, they are a very small group out of 000's of players.
Civ is supposed to be a game of
Leaders, not
lawyers. IMsemi-HO