Originally, Flux was used for forging, but is now also used as an extremely common currency. The other main currency used on the Marketplace are Penta-Forged Shadow Souls.
There are many ways to earn flux:
200-500 flux range
1k-5k Flux range
In the mid game, you will need lots of flux for forging, and other needs such as getting the all powerful Emblem, Surestrike. If you want to make thousands of flux per hour, then you could go mining, earning ~3.5kf depending on how fast you find caves, and ores. You can also work for building clubs, depending on how good of a builder you are, you can get paid 2-8kf per hour. Another, however expensive, strategy is to get a large amount of Magic Find and go dungeon crawling, with a high enough magic find number, you will be able to get a large amount of Shadow Level 1 or 2 and you have the possibility to get Shadow Levels 3+.
15k+ (More like 40k+)
You are now practically rich now (Big emphasis on the wold practically), but not, you need forging items like "Purifying Dragon Flames" and "Penta-Forged Shadow Souls" and maybe even costumes just for fun.
Now, there are many many ways to grab lots and lots of flux, a good way is to keep up with all of the above objects, even though they are listed for different levels of flux, they are all equally important to do and can each give you the amount of flux needed. A few things before I show you the secrets to how to make yourself rich, first of all, remember to dungeon crawl and farm the biome materials like Somber Souls or Robotic Salvage, they are still important.
Alright, first of all, do weeklies, this is of course the Shadow Tower, the Battle Arena portal and your Legendary Tomes, The Shadow Tower grants Titan Souls for each of the bosses defeated, once a week for each boss, depending on which boss, you can get anywhere from 1-5 Titan souls, except for the Daughter of the Moon, which gives Lunar Souls but we will talk about that later. At the Shadowy Market, you may trade these Titan Souls for Titan's Treasures, which each gives you a varying amount of flux from 4500-9999, they can also give 2-5 Megaflux Tanks, if you're lucky. The Battle Arena grants you 1 Classic Battle Box every 12 matches you play (Exactly 60 minutes - 30 if Patron), this can be sold for a decent price but be warned, don't set your selected price too low or else the price will drop, meaning that when you sell it again, you will get less flux than before. Battle boxes have a limit based on your Battle Factor but I recommend doing 2 battle factor, or 4 battle boxes, every week if you don't want to exhaust yourselves over PvP. Finally, the Legendary Tomes, Legendary tomes are like regular tomes but they only activate once a week, before going to sleep again, the Lunar souls said above can be used to make 2 Legendary tomes, the Ropey Readings, giving 20 Tentacles of Q'bthulhu per proc, and the Oculus Opus, which gives 400 Eyes of Q'bthulhu per proc. Other Legendary tomes include the Chaos Codex (10 Chaos Chests per proc), Dragon Coin Digest (25 Dragon Coins per proc), Pyrotechnics Papers and the best of them all, the Writings of Wonder (Giving two highly expensive Troves of Wonders per proc. Proccing so many of them may seem like a chore, but the two very best ones you can activate is the Chaos Codex and The Writings of Wonder, as they give you the most flux.
Simply do whatever is listed up there, you can be at 200k flux and you could still be farming Robotic Salvage to sell, or you could be scraping the barrel and have 5 Chaos Factor, whatever your choice of earning flux is, it will get you rich.