Hey,

To begin I want to say that I have completed very in-depth and extensive playthrough of Horizon on V. Hard. It took me 70h, never had shortage of materials or potions and was sitting on 61k shards as credits started to roll. I don't mean this as a brag, but just to assure you that what I'm about to say economy of Horizon has some merit to it.

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What first?

I would recommend that you go to the Daytower and talk to merchant there. Check his stock of weapons and outfits, decide what weapons you want soon-ish and what outfit is your goal. Stats on outfits mean less than one would think so fashion over stats? I picked Carja Silks Heavy when I came to Daytower for first time and used only that for rest 50-ish hours.

When you have decided what you want to aim for then make note what Hearts or Braidings is required. For e.g. outfit I mentioned needs 1x Longleg Heart. These requirements play into economy control.

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What to sell 100%?

Desert Glass, Slagshine Glass and Ancient Stuff are all vendor trash.

Nothing of value can be obtained with those items from vendors so you can sell all stack of those three items. If you really want to play it safe hold like 10x Desert and 10x Slagshine Glass, but that is 2 inventory slots wasted.

Consider selling all stacks of easily farmable / obtainable hearts and lenses, if not needed for weapon or outfit on your get-soon list. Machines like Watchers, Grazers and Scrappers are very plentiful, fast to kill and drop their core materials quite often so you can farm those back if and when needed. Rarer the machine type or more difficult to kill keep its hearts and lenses in inventory, but never more than 5x. Even 3x stacks are large enough.

Once you obtain permanent fast travel pack you can sell all other fast travel items.

Lower tier weapons. If you have e.g. purple sharpshot bow there is no need to carry about blue and green ones of that.

Lower tier mods. If have e.g. purple damage coils you should sell all blue and green ones. Here I aimed to have 2x to 3x of each type on me. Sold rest. So if I had 5 damage coils I would keep 2 best, sell 3. Extra income can be generated by swapping newer and better mods into the gear and then selling old ones.

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What to sell in moderation?

Metal Vessels, Sparkers, Wires, Blazes, Chillwater, Echo Shells and Metalburn

All those items above should be sold when passing personal minimum threshold. I always aimed to have 200x everything else, but 200x to 300x Wire and just 50x to 100x Metalburn. This way always had good amount of ammo crafting materials at all the times, but because how easily these items also stack up you make good money by selling e.g. 100x Blaze when sitting on 300x.

Most of hearts and lenses can be sold fully, but if you want to be on safe side hold on to 3x lenses / hearts. 5 is maximum stack size, but you never need more than 3 at the time for purchases. More rarer and harder machine is then hold on to lenses and hearts.

Flowers are for crafting potions. No need to have more than one full stack (25x), but I always kept those stacks at 15. Sold everything over that.

Same with meats as with flowers.

Animal bones and skins, if you are okay with refarming these parts when gaining new pouches you can sell all these for now.

Braidings, sell everything over 3x or 5x. I kept 3x stacks.

Traps under "Traps & Potions", depending on how you use them keep e.g. 6x of each or sell all. Steady income here too.

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Anything else?

I never went out of my way to kill all machines I found or hunt down every chest in area, but as you come by that stuff just loot it if it isn't nailed down. I know you do already, but just saying.

One thing you will never run out of and is very easy to come by is ridge-wood. It can't be sold to vendors and if you have even 2k of that stuff don't go out of your way to get more. Inventory filler.

As you trim down on what you are carrying and how much your income will skyrocket as you are constantly selling stuff, but also gain more inventory space making it unlikely to hit that 100 / 100.

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Longer than I expected, hopefully was somewhat coherent and helpful read.