I saw in some gameplay videos that some upgrades or missions require rare components. Is there anything in particular that I should hold on to for quests or crafting?

Hold on to blue (rare) stuff and you'll be fine. Even if you needed something green (uncommon) at some point it should be fairly easy to get. Although honestly, from what I've seen so far you could probably sell everything and just make note of what you need from the store.

@ares42: Thanks. I'll probably sell most things that aren't blue.

There's also a buyback mechanic in stores, but I'm not sure if it's limited to the last x items you sold, or if there's a time limit, or something like that. From the little bit I've played so far, it does seem to be cross-store - so I think you can sell something to Bob, then buy it back from Alice.

@alistercat: It's too numerous to list. Mostly keep things like Watcher lenses, Sabretooth hearts, etc as those are needed to buy new armor and weapons along with currency. Other than that man it's so much stuff.. lots of different types of animal meats and parts are used for expanding your inventory and ammunition pouches.

They crammed a few too many of those types where I've been killings boars for ages hoping for that boar bone I need and I keep getting skins and trophies and meats but no bone! Damn you boars.

I think its okay to sell anything that your inventory marks as "Sell for metal shards" and Only that message. A lot of resources will say that message as well as others like "Trade with Merchants" or "Used for Upgrades" and i haven't been selling those.

But things like "Animal Bone Charm" or ancient watches will only have "Sell for metal shards" (or the message is something close to that) and I THINK their only use is to sell, I don't know for sure, but it makes sense. It was a little confusing at first, since a lot of items include "Sell for shards" in their description as well as other descriptors.

I've been keeping any blue or green items. Animal skins and bones, machine hearts and lens and whatnot.

I have a rule where I only keep two full stacks of any object, otherwise my inventory explodes (even with maxed out capacity). Except wire. I always need more wire.

As others said, sell green stuff, keep blue stuff (for the most part) and keep an eye on what gear you're working for and what your preferred ammo needs for construction.

Don't forget that if you need to collect resources for a specific item from a shop, or upgrade of a weapon/carry capacity, you can create a quest for it so you don't have to worry about remembering something you specifically need/want.

@p1x3lch1ck: I wish more games did this. It's an awesome feature.