With games like this, I always like to know which items I should keep because they might be useful later on, and which items I can safely sell because they are useless or because they are used for things my character build won't need. I collected all this information while I was playing the game and now that I've finished a playthrough, I decided to post it as a guide.

Note: this guide doesn't list where to find each of the crafting recipes because there are already a couple of guides that list that information. I suggest https://guides.gamepressure.com/elex/guide.asp?ID=42005. Do keep in mind that some recipes are only sold by the merchant if you joined their respective faction. For example, Instructions to make Charmer is only sold by the Goods Trader in The Fort if you've joined the Outlaws. Instructions to make Ring of Resistance is only sold by the Armor Smith in Goliet if you've joined the Berserkers. And so on.

Sometimes, a recipe's name contains a different word for the item than the actual item's name. I always put the recipe's version first and the name of the actual item behind it between brackets. For example, the guide mentions "Elex Potion (Elex Drink)" => that's because the recipe is called 'Instructions to make Elex Potion' but the item in your inventory is called 'Elex Drink'.

Notes:

• The Good Eater perk doubles the healing values (but not the other buffs' values) so for example, with Good Eater, the Strong Bone Marrow Soup heals 340 HP instead of 170, but it still only adds 10 armor bonus.
  
  
• Personally, I liked the Bloody Burger and the Strong Bone Marrow Soup a lot because they're very good items and very easy to make. It just meant buying all the bread I came across to be able to make all the Bloody Burgers I needed, and keeping all the Large Healing Potions (or making some when needed) for the Strong Bone Marrow Soup. I also always made all the High-performance Soup I could make because there's no other use for Energy Drinks and Energy Bars anyway.

Note: none of these are really powerful compared to the jewelry you'll find while questing and exploring, so I don't advise spending a Learning Point to learn the Goldsmith skill. Some people like the Ring of Defense for a melee build but I prefer the Ring of Toughness (+20 Maximum Health and +10 Maximum Stamina) anyway.

Notes:

• If you're not an Outlaw, you get a 50% penalty on experience gained while under the influence of stims.
  
  
• I didn't play Outlaw but for difficult fights I still liked to pop a Pick-me-up, Mind Changer or Tough Guy when I had them.
  
  
• Everyone can craft stims provided they have the Chemistry skill, so not just Outlaws.

Note: I only listed the stationary NPCs, not the travelling merchants. Also, I did not list the parts needed for crafting ammo because they are so common it would make the list ridiculously long. Someone did ask where to find Plasma Cells so I'll mention here you can buy them from Hagen at The Hort and from the Trader you can install in Origin.

Besides for creating items, you also need materials to upgrade weapons. The ingredients vary with the type of weapon:

• For upgrading melee weapons you need Mana, Iron Ore, Gold Nugget and Dark Leather.
  
  Note: I read in a forum post that Troll Heart and Stalker Ear are also required to upgrade some weapons. I can't confirm this but I thought I'd mention it to be on the safe side.
  
  
• For upgrading bows, you need Mana, Iron Ore, Sulfur Nugget and Bone.
  
  
• For upgrading other 'old-school' ranged weapons, you need Iron Ore, Sulfur Nugget, Metal Scrap, Tape, Hose and Fine Dining Knife.
  
  
• For upgrading 'high-tech' ranged weapons, you need Natural Elex, Electronic Scrap and Mental Energy Reserve, then Strong Mental Energy Reserve and finally Powerful Mental Energy Reserve.
  
  
• For upgrading shields, you need Iron Ore and Mana. To upgrade the Energy Shield, you need Mental Energy Reserve, then Strong Mental Energy Reserve and finally Powerful Mental Energy Reserve.

If you're a Berserker and want to enchant melee weapons with Fire, Poison or Ice, you need:

• Mana
  
• Mandibles (fire), Chitin Armor (ice) or Entrails (poison)

If you're a Cleric and want to enchant melee weapons with Energy, PSI or Stasis, you need:

• Natural Elex
  
• Electronic Scrap
  
• Iron Ore (energy), Sulfur Nugget (PSI) or Gold Nugget (stasis)

If you're an Outlaw and want to enchant melee weapons with Bleeding or Radiation, you need:

• Iron Ore
  
• Metal Scrap
  
• Hand Grenade (bleeding) or Radioactive Shell (radiation)

While questing, NPCs will ask you for the following items:

• 50 Cultivator's Loaf
  
• 6 Dried Meat
  
• 1 Canned Food

It's 100% safe to sell all the items in the category 'Junk' and 'Valuables' except maybe for:

• Old World Coin, which can be used to purchase food and beverages from the vending machines in The Hort.
  
  
• Fine Dining Knife if you want to be able to upgrade shotguns.

Next, from all the above information, I drew the following conclusions for myself as to what other items are safe to sell. These are items you'll loot a ton of if you invest in the Animal Trophies skill, which I strongly advise you do if you want to become filthy rich in Elex.

• Beak, Pelt, Ivory, Raptorial Leg and Ivory Leg aren't used for crafting.
  
  
• Claw is only used to make Serrated Bolt and Serrated Spear, so if you're not playing Outlaw (or if you are but have no need for either serrated ammo), you can safely sell all the Claws you find.
  
  
• Teeth are only used for crafting Piercing Spear and Warrior's Amulet. So if you're not playing Outlaw (or if you are but have no need for those spears), you can either sell all the Teeth you find or just keep 12 of them if you intend to make the amulet.
  
  
• Feather, Wing, Cyclop's Eye, Ivory Plate and Troll Heart (*) only seem to be used for Goldsmithing. Since I don't think Goldsmithing is worth it in this game, I sold all of these items.
  
  (*) As I mentioned above, someone claimed Troll Heart is used for weapon upgrading. I never encountered a weapon that required a Troll Heart to upgrade but I probably didn't check every single weapon in the game so to be on the safe side, you may want to hold on to your Troll Hearts.

You can draw other conclusions depending on your build.