With trading contracts, much like the trader works today- the trader offers a deal and gives you a limited time to complete the task. If the task is completed then you gain more trust with the trader and can sell more expensive and rare goods, and fill a higher quantity of bulk orders.
Bulk and Rare
I think there are two main categories of trade- bulk and rare.
Trading items in bulk would earn you more gold of course, but would also give you favor with the trader with increased discounts to future items you buy from that trader, and would be for orders of sets of furniture, baskets of food, bundles of cloth and leather, carpets, plant pots, potions etc.
Selling rare items such as armor, weapons, food crates, gold, silver would give you increased profits with that trader.
There would have to a be a set limit where the trader can offer no more profit and no more discounts.
Finally a quest will be unlocked that will allow you to open your own trading network where you can make 100% of the profits and decide on what to offer in the store.