Has your inventory been filling with items and you aren't sure if you need them, or what volume of items you need? Then this is what you might be looking for. There are more in depth articles, google docs, guides, and wikis for resource uses, but I couldn't find a quick list of things to hold on to or sell. Hopefully this will assist you.

• Animal parts will list amount needed for carrying capacity upgrades, weapon trades, outfit trades, quests, and how many will be needed for miscellaneous trades.
  
• Machine parts will show weapon and outfit trades, quests, and if it is used for trading for boxes from special vendors (As they're repeatable in most cases, it will show a YES/NONE.)
  
• Amounts will be in total amount needed for the entire game, but will also show the value you will need for Ultra Hard difficulty. Like so: 2 | 4 ... the latter value will be the increased cost of UH.
  
• The amounts also include NewGame+ items that are not available during your first playthrough.

Spoiler Warning: I will not be hiding machine names except for two story machines that only appear in rare occasions or story missions.

These items can be traded for nothing, crafted into nothing, and hold no other value. The item may list it as usable with merchants but it is not. The DLC Frozen Wilds doesn't use them either. They're in the clear to be sold.

• Goose Bones
  
• Strider Lens
  
• Sawtooth Lens
  
• Bellowback Lens
  
• Charger Lens
  
• Stalker Lens
  
• Corruptor Lens
  
• Deathbringer Lens
  
• Broadhead Heart
  
• Glinthawk Heart
  
• Ravager Heart
  
• Corruptor Heart
  
• Deathbringer Heart
  
• Any Ancient Item
  
• Any Animal Trophy Item

Guides I've found useful:
Inventory Management Guide - By Brahms
Horizon Zero Dawn - 100% Achievement Guide - By Brachragon
Bluegleam Checklist - by u/EruditeAF

Horizon Zero Dawn Wiki (Beware Spoilers!)
Horizon Wiki[horizon.fandom.com]

This is my first guide, I completed the game at 143 Hours with all achievements (except the NG+ & UH) and wanted to share what I had put together. I haven't started my NG+ Ultra Hard playthrough yet, but that's next on the list. If any information is incorrect let me know and I'll look into it and fix it. Thanks for using the guide and I hope it helps!

Guide Version Info:
v1.0 - Initial Guide
v1.1 - Tagged DLC Machines & Animals, left the tag off since Steam comes with the DLC automatically but non-steam users might find the guide. Added Bluegleam checklist link to useful links.