You should buy items that will help further advance in the game, such as items that help you awaken and items that help you upgrade equipment. The only time you should buy frags, if it the ninja is worth it, or you are just flowing with lots of money and want to collect that ninja.

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 :-)

i say buy all frags and materials that sell for coins only, never waste coupons or ingots ( those are overpriced + its better to save coupons and ingots for special events). otherwise coins u can waste as much as u like, even if u have a ninja to 5 stars already is often worth spending 50-60k coins for 5 frags that u can still refine for points to further upgrade iruka.