Frowned upon? Against the rules? Whats the server's stance on this issue, and what does everyone else think?
If you are referring to selling MapleRoyals Mesos for real world money, yes it is against the rules. You would know that if you read the Terms and Conditions. Which you should have read since you agreed to them when you registered an in game account.
It's number 18. "Exploit the Software, Cash Items or the game for any commercial purpose, including the provision of "power leveling" services; Punishment: Permanent ban"