How would you envision this working? Some event where if the AI has >= a certain gold threshold and owns a city with an international port/coastal flag they may purchase a unit that's for sale and have it shipped there?
Something like that. You said that instead of buying units, the AI was simply given them. My idea is to turn some of these gifts into purchases, perhaps using if statements so that the AI buy from the lowest bidder in a class of equipment, or at least has an increased chance to do so. Otherwise, the weapon sales mechanic doesn't mean much to the European player in single player.
On the subject of European weapon sales, would you like me to change the code so that pro-west can only buy the cheapest equipment offered in a given class? Otherwise, Pro-West would nearly always buy from USA, the only exception being a type of unit not being offered by the USA, since the USA player controls the pro west tribe.