Question about trading for those experienced in Uncharted Waters 2

There are several famous trade routes in Uncharted Waters 2,
and when I was playing heavily, I only ever did Athens-Istanbul.
I tried some other recommended trade routes recently,
but I immediately ran into a problem: market prices...
For example, the London Iron Ore - Pisa Silk trade is famous,
but since Iron Ore and Silk belong to different categories,
if you trade them back and forth, the prices for both become extremely expensive. T_T
The Athens-Istanbul route didn't have this issue because it involves trading between the same luxury goods category...
When trading items from different categories like this,
do I have to go and buy other items of the same type as what I intend to sell
just to lower the market price every single time before I sell my goods?
I look forward to hearing from the experts...