Because this question is too frequently asked. & let's face it, we could all use a new idea or two.
Included is a compilation of ideas gathered both from the guild, those around me, & from my personal experience - I take no credit for suggesting all of these ideas. If you have any ideas you want to add to the list, let me know & I'll add it in. (:
Non-skill dependent:
- Sell holy water, 20k per stack. Even better if you score a personal shop spot beside Ch1 Taillteann altar. It's the universally easiest way to make money in the game, but also can be one of the slower ways to do so if you have Beginner-level in the Church/Healer part time job.
- Got gems? (1k~2k/cm; diamonds are 3k~5k/cm?) Be it ego food or gem upgraded joys, gems are apparently profitable. Dependent on size.
- Alby enchant spam (~7k/enchant). If you see a party up, or know someone ranking enchant, try passing a note to see if they're buying enchants. Most profitable during 2x Training / Double Rainbow events, but can also work as long as you know someone who's immediately buying.
- Gather adventurer seals (varied). From the smaller things like enchant expiration scrolls, to the ever amazing Royal Knight set, this is one of the easier ways of accumulating income, although it may take a while depending on what you want to get.
- Buy low, sell high. Requires some knowledge of current market prices. Can be a really great way of making money if you are either lucky, or put enough time into wandering Dunbarton/sifting through party chat all day. Selling rare event items when they are first released also works very well.
- Afk fishing in Avon (10m+, but good luck). Maybe you'll land that Rosemary Glove pattern. Just maybe. But don't rely on this too much.
Combat Skill dependent:
- Rafting & selling Teeth-Patterned Ankle Boots (4k/boot). Most efficient with r1 Thunder for the 4-hit-on-first-charge, but r5 will suffice. A crystal lightning wand with casting speed upgrades is recommended. Archery also works relatively well.
- Hot-Air Ballooning w/ the gold reward (17k/run with max. stars). Most efficient with r1 Firebolt & a CC+4 fire wand. Kill the ice(/lightning) wyverns east of the Calida camp before time runs out, then exchange at Kelpie.
- Fomor scroll collection (varied). If you have a bit of room to spare, collecting fomor scrolls (eg. Mimics (6k/stack), skeletons(7k~30k/stack iirc, etc.) can get you some decent profits as well.
- Rabbie spam (varied). Spam Rabbie Adv. for IS2, yes? (Or normal for TH4?) -Thanks Seung!
- Commercing (10k+/run). With decent merchant ratings, commercing 4th and 5th tier items from certain towns can net you some decent profits. Arguably, it's not 100% combat-dependent, but when bandits spawn, you may find yourself in a bit of trouble without some good combat skills. I personally don't trust those invisible bandits even though I spend more time trying to avoid them ...
- Higher difficulty'd SMs & TMs (varied). Look for the gold reward when you select a mission. Some missions with lots of monsters (see: Tara's Sulfur Spider in the Shadow Realm, Fomor Attack, The Other Alchemists, etc.) also result in more gold drops. Don't forget to pick up your gold! Works best with a summoned bag doll (Vocaloid, Animals, Incu/Succubus).
- NPC Tournament (54.5k per complete cycle). Generally for higher-levelled / stronger players. Takes place in Avon.
High-ranked Life Skill dependent: (don't forget the prod. parties & Monday bonus!)
- Potion-making (12k~20k/potion). Mana potions seem to be one of the highest in demand, although HP & Stamina potions are also demanded. With Monday bonus, recommended rank is rB+, else, r9+ is sufficient. If you don't wish to buy materials, access to decently ranked herbalism & a herb farm is recommended.
- Tailoring (varied). Heavily dependent on new releases for highest profit, although some currently released patterns (eg. Karis Wiz. Suits & Druid Robes) can net you some decent profits. If you don't wish to buy materials, access to high ranked weaving is recommended.
- Weaving (~7k/stack). Thin threads & thick threads sell decently, and are easy to make. A high rank is not required; best sold on Mondays. Related to the weaving skill, picking up fine & finest leathers that drop on the ground will net you 5k~7k per leather. SO PICK IT UP if you can!
- Blacksmithing (varied). Again, heavily dependent on new releases for highest profit, although some currently released manuals (eg. Dustin Armor, Avon Shields) can net you some decent profits. If you don't wish to buy materials, access to decently ranked refining and/or synthesis is recommended.
- Magic Craft (varied). A newer release, hermit staves (r5+) seem to net the best profit, although Savage wands (r5+) net moderate profits as well. If you don't wish to buy materials, access to high-ranked Shyllien Ecology and a carpenter makes the gathering of materials more efficient.
- Hillwen Engineering (varied). A newer release, Dowra SE's & Taunes MK's (r5+) seem to net the best profit. If you don't wish to buy materials, access to high-ranked Rare Mineralogy makes the gathering of materials more efficient.
NX to blow?: You're so lucky ... T~T
- Dye Ampoules (180k~200k/each).
- Fine reforges (200k+/reforge or 7m+ for 35), Credne's reforges (500k+/reforge).
- Well-sought gachapon items (varied) might work. But it's purely luck based.
- Newly released character card outfits (and reg. dyes in conjunction with said card releases) also net the best profits within the first few days ~ week of being first obtained (dye prices will stay up there for longer).
Or invest!:
- If you have some money that you don't need immediately, buying items that are lower(ing) in price, but would generally be worth a lot more to people if they weren't so widespread, can potentially net you a lot more in the future. These may include (well-enchanted & r1 reforged) equipment, premium card outfits, some of the better dual-enchant scrolls, etc. You generally want to choose items that are difficult to obtain or unobtainable through in-game means. However, do keep in mind that these are long term investments, so it'll be difficult to gain a good return on them in the short run unless you bought them very cheaply. Also keep in mind that this is a gamble in its own way - if Nexon decides it wants to release another gacha including a sea of these items, I can't help you, sorry.
There are a lot more ways to make money, but those are just the few ... several .. I remember at this point. Have a suggestion? Feel free to reply / tell me and I'll add it in. Hope it helps. (: