I always buy the awakening materials for coins since you always need them. Actually i buy like 80% of the ninja fragments too just to get them 5 star in future. I would not recommend to buy anything from the blackmarket for coupons or ingots. Also don't buy any food, you will get enough food for just playing the game.
There are materials for equipment upgrading too, but you wont need some of them anymore. If you need Jades in future, just buy them - you will need them :-)