PHB page 144 in the "Selling Treasure" section in the "Arms, Armor, and Other Equipment" paragraph:
Arms, Armor, and Other Equipment. As a general rule, undamaged weapons, armar, and other equipment fetch half their cost when sold in
a market. Weapons and armor used by monsters are rarely in good enough
condition to sell.
Per RAW, this means such items can be looted and used, but have no resale value.
End of the short-&-sweet "100% RAW" answer.
That being said, also as per "RAW", the game is supposed to be, you know, fun?
Assigning a value of zero for what players often feel should be clearly very "typical" loot, that is not "fun", but "frustrating". And I am definitely not alone in feeling this.
It just doesn't make any sense, and can sometimes even feels as if the DM is imposing an "antogonistic" universe. Because, why should captured weapons be so totally and utterly worthless?
Not everybody is freaking rich and able to buy brand new stuff all the time. That family chief lording it over his farmstead of 30 people in back country, fearing recent goblin activity increases, wants to equip as many of his people as possible, right? He won't care about rusty dented swords and smelly patched up armors: as long as they work ok, he wants them! And he'd rather be able to buy several old weapons, than a single brand new one!
Same for a lot of citizenry that care a lot more about usability than looks reputation or prestige. A lot of people would rather save half price on some cheap dagger, than buy a brand new one. "Ooooh it's very rusty you say? So it's not safe to cut my meat with it? No problem because I'm going to use it to defend myself, not for making me samwiches! Ooooh but anybody I hurt might catch tetanos disease you say? Cry me a river, please? Just sell me the dang cheap dagger already."
Sure the cheap stuff will probably last a lot less long. Adventurers might need the best longer-lasting quality stuff, able to withstand fight after fight after fight. But for more ordinary people where if they are lucky they WON'T enter into a fight, ever, and if not, these are the kind of people that will still be glad that their cheap weapon worked just long enough to save their ass, even if only once.
So there should definitely be a "second hand" used weapons & armor market, and thus a few bargain stores here and there for them (in the less uppity parts of town of course). But noooo, as per RAW there can't even be "used weapons backstore bargain bins" anywhere.
RAW? Sure. But... making any sense? Please check out your brains at the enntrance.
This is just an instance where 5e went overboard with oversimplification. That is just a a rule that can easily penalize the fun instead of helping it.
I just do this:
[a] Looted arms have a short list of "quality" going from "New" to "Crap", determining ease or resale, and resell value too (and the worst quality stuff gives combat penalties).
[b] Players seem to care more about frequency of treasure, than actual amount per event. If they get one big whopping treasure once, but every other encounter they can get only next to nothing, they become frustrated, not happy. If the quest giver gives them a big end reward but on every other aspect (cash advance, free equipment, services, hospitality, etc.) he is a penny pincher that forces them to pay for almost everything by themselves, they'll end up hating the guy instead."We save his ass and yet he treats us like easily dismissed mercenaries not like the heroes that we are". So I lower "official module" treasure from the treasure hoard & the final quest reward, to increase the gold value of overall looting and quest-giver freebies.
[c] Instead of making enemy equipment worthless in value, I give them less "costly" equipment that has some resale value (albeit noot half). Since it often has a smaller combat value I just add a few more monsters if necessary.
[d] Replace some humanoid encounters with non-humanoid ones. Maybe combined with a terrain hazard. For example, instead of 3 frakking consecutive and samey-feeling goblin ambushes in the forest, one event is a quicksand patch home to some amphibious beast, another might be an angry mother bear attacking what she perceivbed as a threat to her child, and finally the goblin ambush.
[e] Replace some of the humanoids in the encounter, with non-equipment creatures that make sense. Say, instead of 6 goblins in the sewers, why not 3 goblins plus 6 giant rats? Same challenge, less lootable items.
The RAW Ruling for looting arms can be perfectly fine for a group that just wants to go quickly from fight to fight "video game style" without loosing any game time managing looting enemy corpses. But that is not a one-size-fits-all approach always good for all groups. Check what is fun for your group!
So, ultimately, Rule Zero: Adapt the rules as needed.