Actually, you are wrong about "getting scammed". This is publicly available information that C-Grade items can be bought both in the donation store and in the in-game one for adena (we have announced changes on the forum).
I'll explane you why buying C-Grade in donation store is a fair enough purchase (its advantages compared to buying in the game store):
In our shop, the C-grade is sold together with other useful items such as: runes of experience, mount, samon bracelet, buff cocktails, shots, and so on. Some of these items are unique and cannot be purchased separately.