Question same as the title - Do CS's sell units that you gift to them? Is it because they also pay unit maintenance?
Haven't been able to find this on the forum yet.
I seem to notice that if I gift a CS more than 2 units of any non-infantry unit, the additional ones will disappear or never arrive.
As far as I can tell, gifting units to a CS that hasn't requested them is fairly pointless. You don't get a bump in your influence over the CS and the units go to waste. Even if a CS has requested units ("I'm being attacked, please send troops!") you still don't get any more influence over them if you respond. You do, however, get the pleasure of wearing down the attacking Civ without any diplo penalty.
The CSs are, to me, mathematical constructs put there to add variety to the game. Their main purpose in-game seems to be to provide both the human and AI Civs a means of trading Gold for food, culture, units, resources, beakers and, if you fill out the Patronage tree, Great People. Their secondary purpose seems to be to give Civs something to fight over. If I find myself in a game with Alex it's nearly certain that at some point he will attack one of my allied CS and send me a message to the effect that "I've just attacked one of your friends. What are going to do about it?"
If you are allies with a CS it will eventually gift you with resources even though it may only have one instance of a resource. Likewise, I have seen an allied Militaristic CS continue to gift me with units even though it was under attack by an AI civ.
EDIT: I just took a closer look at gifting units to a CS which asks for help. It turns out that you get 2 points of influence for each unit that you gift. Unless you have some rubbish units that you want to get rid of it seems cheaper to just bribe the CS.