Well the cheapest way is to craft and then crystalize your items... I've seen D crystals for 720a (Bartz) :/ hell of aloot.. and C for 3200.
There are some quest for C grade armor like this one ----->>
Onyl 1 time though :/ You can always buy D/C armors in lux/armor shops in giran and then crystalize it.
I suggest if that dude is selling C for 3100 you sell it for 3050 ;D and shout for it!