Rivens are the purple special mods you unlock "Veiled" from Sortie missions found in your navigation console. You can either sell them "Veiled" (not knowing what weapon will they go for) or "Unveil" them by doing special/random objectives in missions or free roam. Then you find out what weapon riven you just unlocked and you have a greater chance for the very expensive one.
Up there I left you a screenshot from official riven market, most of those rivens are not worth 1k fg but they still find buyers for 16k
(there are many for 20, 30 or even 80k but those are just jokes), highest sold riven was 28k as I remember right, trade was somewhere on official forums
Oh and about credits tax, the only high credit tax (1 million credits) comes when you want to buy Primed mod.
Sure Im in no rush, you know where to find me This post was edited by Shoxu on Apr 3 2019 04:25pm