Making gold in ESO is my forte. I wish I could say the same for real life, truly, but alas. The good news is, unlike real life with its nepotism and barriers anyone can become a millionaire and own a lavish manor in Tamriel. Anyone! Including yooouuuuu.
Spiel aside. 😂
I have a few favourite zones I like to visit depending on what I'm after the most.
Rivenspire is fantastic for runestones, butterflies and torchbugs, has one of the most in-demand overworld sets in the game (Necropotence), and has a lotttt of treasure chests. It also has a fantastic route for if you don't exactly want to just roam around collecting things every few paces.
Starting at the Staging Grounds, you have a dolmen you can fight, a delve you can traverse, three treasure chests that spawn in that area alone, and a lot of butterflies and resources to pick.
Herbs, flowers, ore and wood all have a teeny tiny chance to drop the legendary ingredient Aetherial Dust, which at the time of writing this is priced at about 150,000 gold but normally goes for about 80,000. Still. Be rightfully elated if you find one!
I farm every day, weekly, and find maybe two of them.
After that you go east along the beach and through a rocky tunnel, a random enemy from Oblivion has a chance to spawn, then onward to another delve, then a conveniently placed village with a merchant if you need to repair and vendor some junk.
It's at this village you have a choice. You can go along the beach and follow a route that has twelve treasure chests with possible Necropotence pieces inside, or you can go up past the wayshrine for five treasure chests and a lot more resources to plunder.
The beach route also passes a world boss and a delve.
I usually arrive in Rivenspire with the best of intentions -- harvesting nodes -- and usually end up doing the opposite: going on a treasure hunting spree.
Necropotence, if you find divines traited armour or a sharpened lightning staff, will net you a huge chunk of gold. Upgraded to purple quality the Necropotence pieces will get you around 25,000 to 35,000 gold, and the lightning staff? A good 140,000 at least if you upgrade it to yellow quality.
Rivenspire is really one of the best places for all out farming. However, maybe you just want a lot of alchemy ingredients. Lots and lots of alchemy ingredients. Let me introduce you to a place you probably couldn't wait to get away from when you first hit its location in the story. Coldharbour.
In the Hollow City you will find many people just running circles around the whole city. What are they looking for? Herbs!
You won't find any mushrooms here, or butterfly wings or torchbug thoraxes. But you will find a lot of Water Hyacinth, Nirnroot, Columbine, Lady's Smock, Cornflower, Blessed Thistle, Nightshade, Wormwood and Dragonthorn.
Columbine, Lady's Smock, Cornflower, and Blessed Thistle are the ones you need.
They generally sell for 200-300 gold each, and are best sold in stacks of 50 because of alchemy. A stack of 50 will get you around 15,000 gold and it's pretty amazing how quickly you can actually collect the things... providing you're not in direct competition with someone else. Sometimes I find it best to wait a minute or two after someone else has run past and then begin the harvest so that nodes respawn in a more satisfying manner.
Or instead of selling the herbs, you could make your own potions and sell those instead! There are three main potions that sell well:
Essence of Spell Power requires: Cornflower, Lady's Smock, Namira's Rot ~ a stack of 200 potions = 50,000
Essence of Weapon Power requires: Blessed Thistle, Dragonthorn, Water Hyacinth~ a stack of 200 potions = 30,000
Essence of Health requires: Columbine, Mountain Flower, Bugloss ~ a stack of 200 potions = 40,000
If you're more the doing type, you could pop by the Windhelm Public Dungeon -- Hall of the Dead -- and plunder the many, many burial urns located within. You can find rare recipes or style motifs doing this, including the rare Imperial Style Motif which will get you around 100,000 golds.
The WIndhelm Public Dungeon is incredibly easy to solo, including the group event, and even if you're pretty weak there are usually other people in there too, doing the exact same thing you're doing, so you can down bosses with ease with their assistance.
Keeping in mind that although the things you'll find in any urn or container in the original zones is considered older content, there will always be one or two people looking for them.
Yet perhaps you'd like to find something newer and perhaps worth more?
If you have access to Morrowind, Summerset, Elswyr -- YES!
For example, for the budding thief, there is a particular clothier located in Vivec City who has a chance at a rare furnishing recipe sitting in her pocket. A lot of the NPCs who wander around also have stuff hiding in their pockets, although admittedly it is a very low chance and even I, someone who can mindlessly roam around for hours picking flowers, get very bored with the pickpocketing and thieving game after a while.
Always keep some counterfeit pardon edicts in your pocket lest you get caught being naughty, and if all else fails...
And if all else fails - GO FISHING! Pop on some music, sit back, relax, and press 'e' every so often. When you at last have several stacks of fish, fillet them and acquire the occasional Perfect Roe which people are always looking to buy to create the easiest XP Potions. 😊 They're also used for creating a legendary Magicka user food.
Can sell the Perfect Roe for around 14,000 each.
After all of this, the last thing you'll need is a guild trader so you can sell your wares without the hassle of screeching in /zone chat. You'll want to find one with high traffic, after all, you won't get many sales if your trader is one of the random ones sitting by themselves in the middle of nowhere.
Rawl'kha, Elden Root, Mournhold, and Craglorn are where you'll normally find the top guild traders. If you're in with them though, they'll be expecting you to make sales of at least 500,000 a week, or contribute towards trader costs with a substantially smaller donation/raffle-ticket entry.
Other locations are just as good, providing you're in a location with a few other traders. Most people will wayshrine themselves to populated traders to do their hunting and don't bother with the isolated ones unless they're feeling particularly desperate in search of something.
Best of all, when searching for a trader these days all you have to do is walk up to one, talk to it, and apply to join! Woo!
In closing, there are a couple of addons you need to be aware of.
Tamriel Trade Centre has an online search function for the game, but also has a background app you can run in the background while you play the game that informs you of the value of things you find. It's thanks to all the people running the background app that the Trade Centre can get a greater picture of what's available and how much things are worth on average.
Master Merchant has the same feature, but only really works well if you are in more than one trading guild so it can collect more data. I find it handy to have both addons though.
And finally, you simply cannot use the traders in this game without this amazing addon - Awesome Guild Store.
If there was only one addon you could have active, it would hands down be Awesome Guild Store. Zenimax should really implement it into the base game, it's seriously that... common sense and logical compared to the thing they originally created.
All screenshots in this post courtesy of me, 
