Kill things, get money. That's the basis of all gold farming activities. There are ways to do so more intelligently in EE than in most games.
The enemies themselves in EE give fairly little gold per kill. Not nearly enough to get you through the game. However, the drop rates are respectable for basic items, like potions/materials and cheap equipment.
*You can cut costs by taking out weaker enemies en masse to gather potions instead of buying them. Sit when you need to recover. I know at one point I had over 200 each for both healing and mana potions (grade 3) without buying any. I suppose you could also auction large stacks of potions, but selling them won't get you much.
*Terrified thoughts are rare in the wetlands, but become fairly common in VoK. I would assume higher level materials could be found in higher level areas with a similar pattern of frequency. I think 1 gold per was a bit much, but if people will pay it, go for it.
*Equipment can be a major source of gold. You can check the drop list for any enemy within the game simply by clicking it then clicking the little ? button near it's hp/mp values. It's best to find an enemy that drops rings or necklaces as these sell for more simply to NPCs than any other type of equipment. While a weapon might have gained 500 silver or less in VoK, rings and necklaces would fetch 3 gold or so each.
*Keep an eye out for rare drops, with blue or purple names. Even white name drops can have much higher attack or defense than normal, but the chances are higher with green or blue, and purple always has very high basic stats, though low durability. Furhtermore, green will have 2 bonus stats and blue will have 3. This means nothing to NPCs, but they an be auctioned off for a fair bit of cash.
*Don't sit around and let your summon kill things that aggro you. Technically, it's botting and could potentially get you banned. Either way, it's foolish. They can't handle a whole lot solo before dying even at respectable levels and the gold gains are minimal compared to actually doing the work yourself. (credits to koager)
Eternal Eden has many quests and all are worth taking on.
*Main quests can net you a hefty profit upon completion, but are only possible to finish once. They can help you get your skills as you level up, but can't be relied on.
*Repeatable quests take a while to reset, and don't offer much gold, but since you'll probably be farming enemies in the area anyway, you might as well pick them up. In a decent team, you can burn through a whole area's quests rather quickly, gaining a significant boost to your wallet.
*Guild quests can be repeated indefinitely and immediately. Typically, they simply require killing specified enemies x number of times, pretty simple. Also very easy to accomplish in a group. If you can find a required enemy with decent drops, you can keep running the guild quest for extra cash while drop farming anyway(though you will have to rack up kills with other enemies as well). Also note that you can obtain additional guild quests by purchasing them at the town message board in any guild town. The lower the tax for that guild, the cheaper the scrolls and the more money you stand to make.
*Book quests are available at the traveling merchant or grocer for the area. These are simple one time quests that typically get a title when you complete them all, and almost all quest goals parallel main quests anyway, so no reason not to do them. You can buy all books immediately, but must be a certain level to use each one. The best time to buy a book and open it is when you visit a new area and talk to the soul guardian for the first time to initiate the main quest sequence. Odds are the very first enemy you kill in an area is needed for a book quest.
*Forgot to add this earlier, but STACK QUESTS. Repeatable and guild quests often coincide, and pretty much always coincide with main/book quests. It's entirely possible to finish all four types of quest at the same time for a much bigger payday. (thanks to roque for the reminder)
The auction can be your friend. It's really more of a marketplace than an auction though...no bidding.
*Rare equipment and materials can easily get you a decent profit on the auction. Just check the average prices for the items you want to list and base your price accordingly. Cheaper prices mean faster sales, if you need the money immediately.
*If you have the scrolls, you could buy up or craft rare equipment, upgrade it, then put it back on the market at a higher value. It might be better to just sell the scrolls though, unless it's an exceptionally rare item.
*Crafting can be lucrative, if you are lucky. Crafting isn't as quick and easy in this game as one would hope. The material maps cost more than reasonable, and the recipes cost a small fortune in some cases. On top of that, both gathering and crafting can fail, and if a craft fails, seems you lose everything. That said, crafted items offer various bonuses which can be highly desirable. They are also collectible via the archives. If you plan to craft to resell, only craft the highest level gear available, as the lower level gear is easily outmatched even by random drops.
*Cash shop items appear to be trade-able before use, so if you have the real cash to spend, you could resell essential items, like backpacks, safety stones, etc. You can also sell off costumes, pets, mounts, etc, but the market for such items can be less stable and fluctuate wildly depending on new CS releases.
*Real Money Trading: Simply put, don't do it. Buying gold from a gold selling site is against the rules, technically illegal, and just plain stupid in most cases. They can take your info and do whatever they want with it. Most of them are in china, so if they sold your credit card number or something, what would you do about it? Then there's the whole gold farming death camp issue....
If you really HAVE to just convert real money to gold, then buy an aeria fun card and sell the pin. Or buy AP and sell the items in game. Selling the items in the game is the safest way, you can never be certain you'll actually get a valid pin (buying AP in game) or get the gold(selling AP in game) in a funcard IG sale. If you use AP as the medium for converting your actual cash to gold, you're also supporting the game. If you use third party sites, you are KILLING the game.
Anything I've forgotten that doesn't fit in elsewhere.
*Not all skills are necessary to level. You can save a lot of money by just skipping skills you won't use. Personally, I never used any "reg" self buffs on hunter or thief and I still did pretty well. It's easy to level them at low levels, but the higher levels take 2,3,4,5 gold per level +. It really gets insane to level a skill you don't use constantly. (Thanks to zeek for the reminder)
Personal list of high priority skills:
*DoT skills absolutely require leveling up to be more effective. It's expensive, but definitely worth it.
The instant heal skill is essential to level for a cleric
The aggro self buff on defense classes should be a priority. It's more important for warrior than knight, but very helpful in keeping hate regardless.