Presumably when stacking works, none of this will be a problem. It would be nice if bags could hold stacks of coins. The bags therein could be stored in coffers and the coffers taken to the depot. When humans trade coins, you can trade for the bags of coins, or a wooden chest full of bags of coins and so forth.
Addtionally, when economic elements become more prevalent, you could find that your coins will gradually become more valuable over time as your economy grows. That is, if your overall wealth grows higher than the wealth of other civs your coins become a certain percentage more valuable. Alternately, the value of your coin could fluctuate based on the amount of coins available relative to the strength of your economy. That is, if you make a gazillion coins your first year they won't be worth any more than their base material/weight value. Judicious minting can allow you to use coins as a tool for trade as your economy grows.
Personally, I'd prefer seeing traders and travellers behave more randomly and treat the trade depot as a place where you place goods for sale and random traders come and buy goods at their leisure and choice and your broker is given preference settings for whatever goods he prefers to buy and what he is willing to pay for them. That way if a particular trader doesn't like narrow socks, he won't buy any.
[ April 14, 2008: Message edited by: Hague ]