Construction

I see the market for construction is unstable at times, is it worth hoarding while in supply and selling when in demand?

try it

There is no 'good' answer to your question. Every trader has his or her prefs. Personally I wouldn't touch the construction market with a 20 feet pole but I know a trader who earns trillions with buying and selling construction goods...

So the best advice would indeed be to just try it.

What markets are good for traders?

Tobacco - Moonshine.
Honestly - I recall a bread shortage once that almost toppled empires

Trading needs big fluctuations in the price of a product. You would need to look at the price graphs of products to see which ones have a big spread.

Trading can be more profitable than owning a bunch of corps. If you study the market enough, it will tell you its secrets.