One thing you will quickly notice is that Maple Coins is something that doesn't get used outright as you pick it up (since you can't really use it, per se),and can actually be carried outside. What to use them for, you ask. Well, you can trade them with Spiegelmann for stuff. The cheaper option is to trade them
for various level(Maplestory powerleveling) 35-50 weapons(MapleStory Mesos). A list:
Heavy Hammer- 7.
Golden Mole- 20.
Axe Pole Arm-7.
The Nine Dragons- 20.