After a successful fight party regularly checks enemies bodies for any kind of loot - their weapons and armor too with the intent to sell in the city. In 5e PHB pp. 144 you can read:
As a general rule, undamaged weapons, armor, 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.
So from what I understand here reselling enemies equipment is mechanically allowed, but discouraged. As looting is a very popular trope in cRPG games, I would like to also give this option to my players.
However, the PHB rules aren't specific, so I'm not sure how to proceed without being too arbitrary. The equipment rules from 5th Edition SRD also highlights the freedom of what to do for DM.
You can equip Monsters with additional gear and Trinkets however you like, and you decide how much of a monster’s Equipment is recoverable after the creature is slain and whether any of that Equipment is still usable. A battered suit of armor made for a monster is rarely usable by someone else, for instance.
Based on all of this, I want to build mechanic support for DM in line with core rules of dealing with looting weapons, armor and other equipment from slain enemies. Maybe you use or encounter a mechanic which worked fine for your games?
Additional, I want to make sure is that this influx of gold will not destabilize game, by making player characters rich too fast.