in game service for in game money/gold.
if you'd sell arena boosts for irl money,
then it would be illegal.
as long as it's for money that's made in game, it's ...ehm, not something bad.
you are just offering your service to others in exchange for curency ;p
if selling cm boosts, mount boosts, raid boosts etc is smth legit in game
why would this be any different?