Generally from my knowledge in Scania. First thing is somethings in these methods require connections, (e.g. like a good guild and friends) but maplestory is an MMORPG so you need to be able to create connections and meet people.
Black Crystals, Easily mined and go for 2 - 2.5m ea. Medals of Honor from Dojo. Get Link Mules to lv 140 and clear floor 1 each week. Get 8000 Points a Week to get a 10k Honor Medal. Goes for around ~200m ea. Advanced Gollux Scroll Service. This requires you to be trustworthy, so make connections. Service goes for on average 100m ea (some people do for 150m ea) Cube end game gear to unique. Cubes from bosses, Commerci, Monster Collection, Elite Bosses and Legendary Veins. Join Weekly Magnus, Chaos Root Abyss with guild members. Join Magnus if possible, either use the coins to buy boots/belt to use or sell. Some boots can sell for 4-6b Chaos Root Abyss Armour around ~50-60m ea. Weapons vary in price. Elite bossing if you have time. You have a chance for Clean Slate scrolls (save to sell during spell trace fever), Protection Scrolls and cubes.
Using these Methods can get you 1b+ a week easily. Stack this over a number of weeks and you could reach max mesos in a matter of 2 months or less.
Use Owls to check prices!!! PLEASE BE AWARE WITH FLUCTUATING MARKET. PEOPLE WILL CHEAT ON YOU TO GET CHEAP STUFF.
