it's my understanding that bartering resources at the market in pretty much all RTS games operates on the basic laws of supply and demand: flood the market with one resource, and its value will go down, so if you do so much with that in a short time you'll end up having to trade 100 food for 1 metal, for example; i notice this a fair bit late in the game against AIs since their own activities inhibit my expansion out of what i initially land-grabbed earlier in the game so, eventually, i become reliant on internal trade with merchant units and trading resources at the market (since farms at present have infinite food, i use this to do so since, for as little as it gets me, i can get more food much more easily than the other resources) debt would be impractical to fit into the regular gameplay of an RTS game, though. i could see it coming up in a specific scenario with triggers put in place to simulate it where, in order to win you need to pay off those debts (which probably cap at some point for fairness' sake) and periodically rise or else you'll be attacked. stuff like that.