It sounds like you are asking for a basic version of what Rise of Nations: Thrones and Patriots uses. Whenever a resource is sold at the market you get gold for it and the price of the resource drops due to surplus but when the resource is bought the price increases due to deficit. You cannot trade units in that game. Only in real life has there ever been loaning of armies or navies but it would be rather difficult and unnecessary unless you could declare yourself a traveling mercenary looking for the highest price (backstabbing at last second because the defender offered a higher price and upfront
There could either be a finate amount of resources or a cap on price but this does allow for some trading. If you need gold and have a surplus of a resource then you can quickly sell it and buy what you need or just keep the gold for yourself at a later time (pay for your army or more buildings, etc.). Doing that would open up a large amount of possibilities, including limited but replenishing resources (gold/silver veins in moutains, copper veins, forests, water (only in hot areas, such as a desert), stone, dirt (early chinese walls, quite effective) etc.). In reality most of this is not replenishable but hey, this is VIRTUAL reality.