Overview
Here is a list of ways to make money. Methods were tested with v.0.5 English translation.
Item Stars
Item Stars usually give you Unknown Things (5G), Lures (100G), Dish Tickets (100G), Mail Order Spirits (500G), Mail Order Spirit DXs (3000G), and Chunks of Something (5000G). Specific worlds and areas will also have additional items or different item tables.
Fish for Gold
If you have some lures in your inventory, talk to the Old Fisherman in the Deep Sea World below the spawn point. For each lure, you'll get a few Small Fish (100G), as well as chances to get a few Big Fish (500G), 1 or 2 Strange Fish (item) (3000G), and either a Valuable Fish (10000G) or Treasure Chests (5000G).
Treasure
Treasure chests can contain items within them, like Unknown Things, Chunks of Something, or gems. While the gems themselves are practically useless in making money, they can be traded at the Gem Trade Building in Ancient World 1-2 Gosho's Stones (10000G) each. The Deep Sea Buildings (all levels) have item stars that have a chance of giving 1-3 Treasures, as well as Lures, Unknown Things, and Chunks of Something. Perhaps the best method of money making is to start at the Deep Sea Buildings 4th level (Teleport Maze), go back to the 3rd level (northeastern area), interact with the item star, go back to the 4th level, and repeat.
Fight for Money
Few enemies, such as bosses, drop money after battles, dropping at least 1000G. If you have good enough stats (preferably at least 1000HP, a good amount of Spirit (maybe 100 or more), at least 300 Attack and Deft (Dexterity), and the Fantasy Symbol), you can farm money by battling Assassins [Step] in Ruins World's school area after interacting with the computer for 10000G each. Make sure to bring healing items.
NPC Items
During your adventures, it's always good to talk to common, friendly NPCs to receive their items, or fight them if you're not worried about your Heart Wound value. The following is a list of items with decent selling prices (drop percentages taken from the Japanese wiki, does not include items dropped by NPCs that are permanently killed off):
Insta-Gold Thing - 1200G: Dropped by each of the five golden NPCs; Mottenka after interacting with a specific statue with Beast of Shura activated in Town World's statue area (south), Manetenka in front of Town World's shop, Kaitenka in Forest World's art gallery area (random chance of appearing), Nottenka in Ancient World's starting area, and Nietenka in Ancient World's Hell. Drop chance: 90%
Figure - 1200G: Dropped by Townspeople in Town World after battle. Drop chance: 80%
3-color Business Card - 300G: Given by Boss and Workers in Town World's shop. While a lower price than most in this list, it's good to gather them before you sell.
Stinky Fruit - 900G: Given by the running Jungle Cat in Forest World's main area.
Flower Seed - 250G: Given by Nurikabeko in Forest World's fork in the world (path to plant area). Another low price, but if you're growing flowers, you may have some to sell.
Gorgeous Flower - 2000G: Picked in Forest World's plant area. Has a chance of growing from a planted Flower Seed.
Fur - 1200G: Dropped by Dogs in the houses in Forest World's path to the plant area. Drop chance: 80%
Shiny Lilac Hair - 1800G: Dropped by Jumokuko in Forest World's maze area and Ancient World's grape cave. Drop chance: 80%
Final Missile - 2000G: Dropped by Security Robo [mk-II] in any area in Cyber World with sliding doors after killing 2 NPCs and 2 Security Robos. Drop chance: 80%
Black Ball - 500G: Given by Lentigo in Cyber World's huge monolith area.
Ore of Heat - 1900G: Given by Beiazeigeru in Ruins World's hot/cold area after activating the switch.
Demon Blade - 27500G: Dropped by Assassins in Ruins World's dark area. Drop chance: 50%
Black Dish Tickets - 5000G: Given by Hungry in Deep Sea World's teleport maze (buildings area, 4th level) in exchange for dishes.
Bone Knife - 2000G: Use a Fairy of Change on a Twisted Bone, which is dropped by Strange Fish (NPC) in Deep Sea World's depths and Bizarre Mermaids in Deep Sea World's path to Nushi. Drop chance of Twisted Bone: 80% Strange Fish, 70% Bizarre Mermaid.
Copy Paper - 1000G: Given by Amabie in Fantasy World's plane area - indoors
Rainbow Tail - 1500G: Dropped by Tsukimono in Ancient World's ruins area. Drop chance: 80%
Scroll - 900G: Dropped by Imuko in Ancient World's bridge to Sudamaou's temple. Drop chance: 80% (seems higher)
Temple Note - 1000G: Dropped by Oni in Ancient World's Hell. Drop chance: 80%. You only have one chance per visit (unless you killed 50 or less entities), but if you're collecting Scrolls and visit Hell, it's a better money option over the green Oni's Knife.