Original implementation:
You can carry 1 good per troop in your army.
Current implementation:
You can carry any number of goods without regard for army size.
Suggested implementation:
You can carry up to 1 good per troop in your army without it being obvious that you are transporting goods. If you have more than this, then you get a "C" on your map icon, beside your army size. e.g., [50] has 50 troops with him and between 0-50 goods. [50]C has 50 troops and at least 51 goods.
The idea being that you can disguise some amount of goods without it being obvious that you are a trade caravan. 50 troops with 50 goods just looks like a well provisioned army. At some point, though, it becomes too obvious to hide and you are marked as a Trade Caravan on the map. This lets small time traders sneak around selling stuff without necessarily being molested while a big time trader will not simply be able to waltz around as a (0) with 500 goods, betting that nobody will bother attacking one lone poor (0) among all the others.
(We might also allow for up to 10 goods to be transported without getting the "C" mark, so that the poor (0)'s can at least piddle around a bit and scrape together some cash incognito.)