Of course you should be able to sell/gift units. I would impose only 2 limitations. The first would be you can give/sell only x amount of units over y number of turns, this would be dependent on the size of the other civ's military and perhaps the era you are in. Otherwise, you would have exploits such as the one below:
India has both incense and gems and will not sell you any, because they are mad at you. So, you call up Persia, who is your buddy, and is right next door to India, and give them 50 Modern Armors. Then you get them to declare war on India. 5 turns later, Ghandi is bawling in one of his three remaining cities, and your luxury box now has incense and gems in it courtesy of a happily grinning Xerxes.
The second limitation I would impose is on nuke trading. This should be severely frowned upon by all nations, though it can occur. If you do it, and the civ uses the nuke, there is a chance you are discovered as the supplier. Severe diplomatic and military consequences follow for you. Only 1 nuke may be traded to a given Civ every 20 turns.