Okay time for that part of the thread where I make big ole guides on various junk and link them in the OP.
MARKET GUIDE/GENERAL MONEY MANAGEMENT
Money is kind've a thing in this game. You need to know how to make it or you will go broke repairing your shit. As a note, pick up all the money demons drop. This will do enough to keep you afloat early on. Not enough to buy nice things in short order, or at all really, but whatcha gonna do. The answer is sell things. The marketplaces in this game are free to use (in real dollars, you need to use in game currency to open one but still) and are manned on their own, you don't need to be there. So there is no reason not to use them.
Okay, first thing you need to know. What to keep to sell, what to chuck out/NPC.
First off gems. Keep them. I don't mean the demon HP/SP restorers, I mean litteral gems, like rubies, peridot, etc. They sometimes don't sell for a ton, the lower end ones can sell for less then 1k in fact, but the upper end gems like diamonds can sell for litterally millions. A piece. You can get gems from dungeons and the yagiya time trial. Trade in five yagiya seals to get a shot at the gem lottery. Gem prices go from 500 to a million (APPARENTLY DIAMONDS ARE NOT IN THE GEM LOTTO BUT STILL) a piece. If you find it in little boxes in dungeons, start at 500 and move from there, raising it or lowering it as you see fit. Obviously if the gem sells to an NPC for more then 500, price it higher then that, but start low is the point.
Quest items. Notable things are Makanda stones, Faded Earring: Garnets, soul shards. That's not a full list but if you see a quest taking it chances are you can sell it. I've seen Makanda stones for as much as 10k a piece, Faded Earring: Garnet's go for about 20k I'd say. Soul shards I personally price in the 3 digits but you might be able to make it into the 4 digits.
Magnetite. The lazy man's choice. You will build up magnetite naturally as you move along as everything drops a little bit of it, and it is valuble to those of us that use demons away from our personal alignment as shit gets pricy as the demons level up, and the magnetite cost only goes up when they're outside your alignment. As a result selling your spare magnetite (for about 2 macca a piece) is a great way to make cash. Don't dry yourself out on the stuff, be sure to keep a nice store on hand, but it sells like hotcakes.
Incenses. They're very useful but can sell for a bit of change. Little common ones like the 1.2x exp and expertise ones sell for maybe 5k? I'm guessing on that but it sounds reasonable. It goes up the longer it lasts and the higher the multiplier. Incenses are also used in the demon rebirth system and demon force system so don't be afraid to raise those prices if they just fly off the shelves. You're on your own for prices on these, I've never done it.
Anything that effects demon happiness. A happy demon is a strong demon, and a happy demon is a demon that can be conned into being shoved into a crystal to be used as a gear enhancer. Cups of twilight, gift melons, etc. Cups of twilight go for about 10-15k a pop. Gift melons 30-40k I want to say? They're very valuble.
IC chips, and presumably all kinds of dungeon pass esque items. Most are sold in NPC shops but things like Suginami (Unknown) plates could probably be sold for a nice sum. IC chips are about 10k, this seems to be a fairly solid, agreed on price in the community. They drop fairly often off of the various giant enemies in Suginami. Never seen an unknown plate sold but I dunno why it wouldn't work.
Crafting professions are another source of income.
Swordsmith and Arms maker are in kind've an odd place. Generally cash shop items are being sold for normal currency that are vastly better then anything you can make (at low levels). WITH THAT SAID, you can make a bit of scratch if you make sure to keep your weapons reasonably priced. Any AC item is going to be super expensive just because real money was spent for it. Your early game gear will be "bargain gear". A five slot item (should you be so lucky) decked out in various enhancements to it's damage can really hit like a truck so don't underestimate this stuff! Just be sure to read up on the crafting process as enhancing items can easily break them. Like, permanantly.
Synthesizers are probably in the best place. Soul stones are always in high demand, and you generally get tips for just doing the stuff as any crafting alt is a pain to raise, and synthers get paid compartively little for their services then endgame swordsmiths and arms makers.
Med Sci I honestly am not sure about. They can make a lot of really handy items, so just selling incenses I think would be a great way to do things, and I think there are a few items that are important to use for summoning field bosses or something? Also medical items, etcetcetc. Could go either way but I think it's a safe bet.
Miscellanious stuff. Denshi Kairo is a huge cash fountain, go into the Home III service enterance, and use the terminal in there, go to "FROM: Steven" and start up. Basically it's a gauntlet, you can go 3 floors at a time and monsters drop an inordinate amount of mags, macca, and exp in exchange for being pretty buff and sometimes multi aggroing. It's a great way to get ahead. You can do it once a day.
That's about it. THIS WILL BE EDITED AS I DISCOVER MORE STUFF. IF YOU HAVE A PRICE, TELL ME IN GAME AND I'LL ADD IT TO THE POST. KEEP CHECKING THIS POST, THERE IS A LINK IN THE OP IN THE INFORMATION SECTION NEAR THE BOTTOM. THANK YOU AND HAVE A NICE DAY.