No, that would encourage gold farmers and worse case scenario: botters. You don't want to see your servers full of bot accounts running around and farming scrolls at ninja exam wouldn't you? If possible, I would suggest an exchange NPC or a features in the game that allows you to exchange your Ninja Scrolls, Cloths, Threads, etc to a more advance version of the item.
Ex:
100 pcs Sages Scrolls > 5pcs Super Kage Scrolls
100 pcs Medium Cloth > 5 pcs Advance Cloth
100 pcs Medium Thread > 5 pcs Advance Threads
This post was last edited by sohaitot at 2017-2-23 20:37

And who says Im talking about trading between player to player? From what I understand from your post is: your'e implying that items that can be obtain free like cloths and threads should be sold for coupons.

But selling items which are obtained hard through game or by spending money and getting 1coin for them o.o give us some coupons tbh dont give us coins

Given that your suggestion has been implemented and threads, cloths, or scrolls now worth coupons, you will witness a huge influx of gold or should I say coupon farmers aka botters slowly taking over the entire server. This means less recharge and less profit for Oasis.