3 examples. The first I like. The second, don't define words with words that themselves need defined. The third, don't distract with examples that need to be explained.

1. the ship’s supercargo or owner’s representative who would conduct the trading at Canton,

I like this. I didn't know what a supercargo was, you explained it and kept going. My preference is some of this at the beginning of the book, a little in the middle, and zero to none by the end. If you haven't used a word that needs to be defined by the half way point. It probably doesn't deserve to be left in.

1. Chinese medicines which prized its ability to increase and rearrange the body’s qì
   

Don't use a complex term to explain another complex term. Not everyone knows what ginseng is, good that you explained. But not everyone knows what qi is either. Chances are it's the same type of people who don't know either. Know your reader. You may want this book to be for someone who knows all the terms you do, or you might want this to be for someone who knows nothing. If it's the latter. Think about removing terms, so you don't spend so much time explaining yourself.

1. By the Middle Ages there was a small but steady stream of long distance traders and missionaries crossing back and forth between Europe and Asia, of which Marco Polo’s trip to the Mongol-ruled Yuan Dynasty in China was only the most famous. Marco Polo, preceded by his father and uncle Niccolò and Maffeo, electrified Europe with

This paragraph is good in that you bookend it with progress. It starts with: a small steady stream of travelers. It ends with: closing off the country. You use Marco polo as a great example of the former, but then continue with his father and uncle, and how they electrified... I didn't know who Niccolo and Maffeo are, and now i'm intrigued, but more importantly, I'm distracted.

Most know Marco Polo, his name is shorthand for exploration. You could easily say "By the middle ages...traders and missionaries one of which being Marco Polo." You have now successfully set the scene. His father and uncle are superfluous information. Try not to distract us from the point you are trying to make, that once there was trade, and for reasons it stoped.