You can trade Riven Mods if thats what you are asking, doubt the market for rivens mods are huge here.
You could check out this site:
https://riven.market/ to sell, price check etc.
Edit: After reading carefully, looks like u are looking for "riven reroll service", not sure how u can make it safe, have to find a price both parties agree on maybe kuva spent on reroll for x amount of fgs then find a mediator or let the person rerolling broadcast the game on steam which is only 1 click and easy to start a simple stream. Not sure tbh.
This post was edited by OwNii on Dec 5 2017 12:53pm