Apprentice here who has played in person a lot so some of this may be more damaging than helpful.
Trade cards, such as 2 yellow to 2 red, can be incredibly powerful for converting spices multiple times. Make sure to use the trade card multiple times if you can to make the most of it - they can be much more powerful than resource cards!
If a player is taking the leftmost card next turn, try building up to buy the second card from the left before they do.
Resting takes up a whole turn, so avoid doing it too much. Having trade cards that can easily up-convert and down-convert gems of different colors continuously will help you to not rest a lot.
You can never have enough merchant cards! Only if the trade cards do not link well to each other, can this be an issue...
Using the multiplier 2, 3 or 4 times adds up. Do it as often as possible even if it means missing out on a Gold Coin. Market efficiency is key. All those trades turn into points eventually.