Here's what I'd like to see, which combines some of joespaniel's, bippukt's, and Pembroke's ideas:
You can trade units with other civs. If that civ doesn't have the required tech, all techs needed to make that unit are given to that civ, and the value of the techs is therefore added to the base value of the unit. (You don't necessarily have to sell it for that value, though.) You can also trade units even if the civ doesn't have the required resource(s). If you do so, the troops won't heal unless the buyer gives some money to the giver (representing buying a little bit of the resources). In this case, the unit has a normal value, because a) it can't heal (without extra cost) combined with b) the civ can't build that unit themselves, so you're the only source. ALSO, you can lend units for 20 turns. (Ideally, you could set the number of turns, but I think 20 might somehow be hardcoded into the game.) In this case, it's the same as completely selling them (except that they cost less, of course), with all the tech/resource stuff applied. If a civ uses its bought units against the supplier, they suffer a rep hit. This rep hit makes it impossible for them to buy units again (except with the human, of course.) It also affects other "trust" issues (ROPs, alliances, etc.), but only a little. If a civ uses its borrowed units against the supplier, they don't have to give them back, and they suffer a major rep hit.
I'm not sure if the AI would be able to handle this, though.