Obtaining Ducats

You should be able to net roughly 20-30k ducats in a single trip depending on the value of your trade goods, their resell value (generally, as the distance between two towns increases the resell value increases) and the storage capacity of your mode of transportation. You can also level up your trade level, which awards seals of belfast and improves your selection of trade goods. In G15S3 you gain the ability to trade with bandits. They'll buy your stuff at a higher price while lowering your trust level with the town you purchased goods from. There's a daily item which can be sold to bandits without lowering your trust level. If you trade regularly, item repairs won't be an issue though it'll take over a month in order to be able to afford something like calabolg.

In the beginning you'll pull in a few hundred to a few thousand ducats each time you trade.