This is a basic guide on how to make zeny when you're still clueless about RO or semi-experienced but starting out on a new server.
Yes, I'm aware of the fact that this guide won't help seasoned players, but I wanted to cater to the newbie.

1. Choose your first char wisely
I'd say start off with a thief, they're cheap and you won't be gulping down too many potions. A hunter is also okay since you get making arrow later on, but you'll have to buy them at the beginning.
I don't recommend swordmen - they're very gear-dependent and you'll have to pot all day, so you've been warned.
Same for WS. Those are awesome end-game classes though so consider making a WS as your second char to get vending, overcharge and discount.
2. And abuse those skills
I bought stuff without discount before and I felt guilty even if only "lost" about 100K. Depending on whether you can log on your merchant fast or not, buying or selling without the skills might be better, but always buy in bulk when you get the chance to get the 24% discount, especially:
orange, yellow and white potions
mastellas
blue gemstones
berserk potions
3. Get an (AFK) homunculus if you can
Make a homunculus, stay logged on while you're doing something else, and rake in the cash. If you need unlimited potions, find a spot in moscovia dungeon. There are two near the 2 stones, just walk to find them. I can't give you the exact location as it varies a little on my serv but just go to the less crowded one. You'll get aloe leaflets (use them to heal your chars for free) and blue herbs (SP!).
4. Start a business, or even an empire
Get autotrade merchants if you can, or stay logged on overnight to vend. Vend anything and everything, and try to overprice some items but sell other items for next to nothing. That way you'll confuse and please the customers at the same time and they'll usually come back. I used to overprice everything and this is something you can do if your server has just opened, but if it's not that new you need to be reasonable.
Also, don't assume that something crappy won't interest someone else.
I was surprised to see that a lot of Bio 2 loots sell well (Saber, Buster, Saint's Robe).
Basically, even if you think you have nothing in storage, make an effort and put something on sale, even for 10K. You never know, you might get rich quickly.
5. Never Ever Sell to the NPC...
But if what you just dropped is sold by an NPC, then sell it. Whaaat?
Ok let's rewind.
Awak potion: sold by NPC
Do you need it for your sage? No. You could keep it for another char but no, you need the money and you don't want to make a new char anytime soon You just killed x monster and you have way too many awakening potions. What do you do? You sell the awaks to the NPC. Easy.
Anything else, you keep. Empty bottles sell well. Rainbow shells are treasures to newbies who want to do exp quests or sages.
Don't sell cobwebs either. The list is very long actually. In doubt, keep the stuff

If someone is buying an item in bulk, it means that they're willing to pay a hefty price to get said item. Unless they're idiots and want 1000 yellow gems for 1M (under the market price and the actual NPC price of the item), sell the stuff to them if you don't need it.
6. Look for quests you can do easily and resell the goods
Rune Niflheim is a Spanish website that explains where all the traditional RO quests are. It's extremely helpful and I wholeheartedly recommend you to visit it. It's better than RMS as you get the picture of the NPC and a red dot representing said NPC on the map, plus some instructions (in Spanish, but hey, it's not that hard to understand).
Stuff like opera masque can sell incredibly well as O Masque is one of the best headgears in the game, especially when you don't have customs.
7. Log on as often as possible, even for 5 minutes, to look at the shops and get a feel for the market price of popular items
That way, you can start buying/reselling -tricky and not recommended but can pay off- and at least you'll be able to spot that merchant who will go bankrupt if he keeps selling stuff so cheap.
8. Make friends with people
I'm not good at making friends on RO. I like the people on this forum, but the people on my serv are a bit too cocky for my taste. Anyway, try to make friends with (powerful) players if you don't want to end up guildless and broke.
Sometimes, asking something in town that borders on begging but that is not actually begging like "Does someone have x item? I'm new here" can help start a conversation and will get you the coveted item more often than not.
Let's be clear here, I only did that a couple of times to get a burning bow on a new serv. It's not something you should do every day, unless you like to annoy people. However, saying thank you and contacting the benefactor again for some friendly chit chat goes a long way.
9. Drop what you can drop alone, but don't try to drop the undroppable alone
Horrible sentence I know.
Basically if you're looking for 200 metal fragments and really don't have time/ can't kill apocalypse, then buy the fragments at a hefty price if you need them.
If you know that you don't want a champ or that you can't farm an item because too many people are killing deviling/LOD/Eddga/whatever, buy the item. Even if it's quite expensive.
However, if you can drop something alone, even if it's annoying, do it. I'm not talking impossibly long and annoying like the metal fragments but mildly annoying like the black cat doll quest or dropping a card.
Zenys are worth a lot, items aren't worth that much. At the end of the day, if a higher-end piece of equipment is available in a shop, if you don't save up, you won't be able to buy it because you squandered your money.
I learned that the hard way and right now I'm trying to avoid spending zeny unless I absolutely need the item.
If you want something that much, you'll buy it in a matter of seconds. If you have to think, it's usually a bad sign (means that it wasn't on your shopping list in the first place).
Hope that helps
