Well...my ingame name is McBurger, so I'm calling this McBurgers guide..heh..
I'm lv30 and can rake in some decent profit for my lv. If I try hard enough, I can rake in 5-10m per week..so bad I'm a sucker for items and blow my cash once I get 5m XD.
My main profit comes from...well..one word.
ALCHEMY/ELEMENTS Ook maybe two words!
First off, you need money to make money with this method, So I advice you to ask your friend for a 500k loan, if theyr'e a high lv, 500k is nothing so it shouldnt be a problem, just be sure to pay them back.
Now, go out and search stalls with gear 3Degrees and above for cheap prices.
Blow around, 380k of the loan on that gear, blow the rest on destroyer rondos. I advise buying mostly weapons and chest/leg gear, from personal experiances..they give the most elements.
Destroyer rondo EACH PEICE! Elements lv 2-4 can range from 500-2k a peice, I advise going 1k-1.5kea with ice elements 2-4, most other elements range from 500-1k, ice is the most profitable element. Destroey ronding stuff can get you around 25-50 of each elements..and all the elements you got from destroyer rondoing the item can easily out-profit what you bought the item you Destroyer Rondoed for, thus making you a good chunk of profit. Also, ALWAYS PICK UP ACLCHEMY ITEMS WHEN GRINDING, such as Acorns, straw, Stone skull neclaces, stolen neclaces..w\e you get the point

And tablets, I advise NPCing tablets lv4 and below, lv5+ tablets..use your elements(If you don't want to sell them) with the tablets to add a blue to good gear.
Buy a +3 or higher, blue it out, sell it for 2 times- Three times the profit.
MERCHING
Buy low, sell high, simple as that..Example:Look for elixers for as low as possible then sell em for high as possible or slightly above average price.
Like this..
Buy weapon lixer for 160k, sell for 200k. 200k wep elixers are hard to sell, but you'll eventually find a buyer. this method is relatvly risky, and I myself don't normally use it..
GRINDING
I'm not talking about EXP/SP Farming grinding, I'm talking about profit grinding, Use the potion size below the one you normally use to save some money, seeing as you should be grinding on Greens when profit grinding. Your grinding on greens(Or at least should be like I mentioned), which means you can kill them with little effort, which means fast kills, fast kills mean...well..alot of kills in a long period of time:P Alot of kills, means a high chance of a weapon/armor drops which you can either blue out and sell, or destroyer rondo..not to mentionn gold drops which if you grind for long enough, can give back the profit you payed for the potions you bought and give you ane xtra 100-300k. (Depending on the mob) If your lucky enough, you MIGHT get an SOS, or a blued out item which can sell for some nice mula.
Another thing I forgot about to mention in the alchemy section..Buy Blue/Clean weapons/items without any + for cheap as possible..and +3 or 5 it(If you have astral) If you can afford to buy a few astrals, +7 it and sell for millions more than what you bought it for. Also, if you bought it clean, add a good chunk of blues on it too, which can add a a good chunk of money more to the original value of the item.
If you bought a Lunar Blade lets say, blued it out, and +5 (With astrals) Or +7 it (DEFINITLY YOU NEED ASTRALS FOR +7) youv simply added a good..eh..6M to the orignal price probably
When blue-ing items out, I suggest making your own elements, yes it will take a while, but will save you loads of cash instead of blowing millions on the elemnts you need. If your filfthy rich, and you can afford to blow a few mil, go for it.
That concludes my guide, any ideas/suggestions are welcome, if I think of more things to add..well..I will.
P.S:Sorry for any Grammar Errors/Typos..I'll meh, proff read it in a day or two/Year or two/ Never. (Jk, i'll get to it within this week..hopefully)