You realize it is forbidden to sell game stuff for real money right? (I am sure in this case they'll consider crypto for real money) If the admins catch you, both the seller and the buyer will get banned. That doesn't mean these things are not happening there but just a friendly warning.
*Can't remember which league I played last time, was probably "delve", It was fun af.
GGG very rarely bans people even for legitimate offenses that are way worse than currency selling. These days you'd have to run a bot farm that sells a few mirrors per day in order to start worrying about the danger of getting banned. The amount that I'm going to sell won't raise any suspicion, people are giving away tens of exalts when they are done with league, I just decided to try to sell it.
But hey, thanks for the warning anyway, if some user will decide to deal with me, they definitely should know that it's a potentially risky operation.