DnD 5e - Selling Treasure
So there you are: standing alongside your allies, victorious among the fallen forms of innumerable foes. Goblins, cultists, minions of evil masters, all lie before you, their various weapons, armor, and other gear all lying where it fell.
Now what do you do with all of it?
Dungeons and Dragons isn't a video game. When you kill something, its belongings don't mysteriously cease to exist. Similarly, they don't all climb into a container for you to quickly collect and be on your way. If slay defeat an evil knight in plate armor, he falls dead but he's still wearing that plate armor. It's right there, and in a lot of cases "finders keepers" applies. Unless you're somewhere civilized like a city with laws and someone to enforce them, if you kill something the expectation is that you and your friends are going to take its belongings.
"Looting the bodies" is a time-honored tradition. Most beast-like monsters won't have anything of value, but you might find a handful of coins or something from previous victims. Intelligent enemies are more likely to have useful items like weapons and potions, and some of them frequently hoard treasure which you can claim for your own. This may seem very violent and unfriendly, but if you just killed the local equivalent of Smaug (go read The Hobbit), there's a hoard of treasure that some monster has been accumulating, possibly for centuries, and you're now the closest thing to a rightful owner.
Keep It, Sell It, or Drop It?
Whenever you find an item, it's helpful to decide what's going to happen to it. Do you or someone in the party keep it? Do you collectively decide to sell it and split the gold? Or is it not worth the effort, so you simply drop it and move on? The decision will vary by item, and an item that could be extremely valuable at 1st level like a suit of armor or a mundane weapon is barely worth the effort to pick it up when you're approaching 20th level and have hundreds of thousands of gold to your name.
While most groups don't closely track weight, try to keep a general idea of how much loot you're carrying around. If it's been a while since you returned to town to sell your new-found pile of pointy sticks, you may be hauling around a wagon full of discarded equipment.
I generally handle this by buying a wagon and some draft animals (mules are very affordable) and hiring some laborers to drive them around, carry loot out of dungeons, and generally manage boring things like that. Whenever my DM asks "how are you carrying all that?" I say "I'm not. That's Phil and Joe's job." Granted, I've had to rescue hired NPCs from a few tight spots, but honestly those have been fun adventures in their own right.
Selling Mundane Items
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.
Generally your own equipment will be "undamaged". The game assumes that your character is maintaining their equipment behind the scenes: sharpening swords, hammering dents out of armor, etc. But monsters are generally assumed to be less invested in the condition of their equipment, so you'll face a lot of enemies with rusty swords, tattered clothes, and dented armor. Collecting a pile of rusty knives from a tribe of goblins generally isn't worth the effort.
However, your Dungeon Master might make exceptions. If you're facing intelligent enemies, especially other "civilized" humanoids like humans or elves, their equipment is probably in good enough condition to be worth collecting. You might find better armor than what you're wearing, or you may just collect a bunch of swords and such to sell when you return to civilization.
At "Tier 1" (the levels ranging from 1st level through 5th level), selling mundane gear is an important part of your character's income. I've done the math on how treasure is distributed, and if you're not collecting and selling mundane gear you're probably not collecting as much money as the game expects. Your Dungeon Master could easily fix this by giving you more money, but they may not know to do that unless they read this site.
Selling magic items isn't as easy as selling mundane items. Anything more powerful than a Potion of Healing is so rare, unusual, and valuable that most people simply can't estimate the value of such an item, much less pay for it. Your +1 sword is worth more than most people's houses.
If you want to sell a magic item, see Downtime, earlier in this guide.
Gems, Jewelry, and Art Objects
Gems, jewelry, art, and similar items retain their full value. You can either trade them for coin, or you can use them as currency for other transactions. When you're dealing in large quantities of money and platinum pieces have stopped being an effective way to manage your money, it can be helpful to carry around valuable gems instead because they're more valuable per pound than precious metals.
For exceptionally valuable treasures, the Dungeon Master may require you to find a buyer. If you walk into a farm town with a painting worth 10,000gp you're unlikely to find a community of laborers with the money to purchase it from you.
In areas far from civilization, many people conduct business through barter. Similar to gems and jewelry but far less fancy, trade goods generally retain their value so long as they aren't damaged or spoilt. Bags of salt, bars of metal, and even livestock all hold their value and they're as good as coin to people whose day-to-day lives depend on those items. When you find yourself in a more civilized area, you can typically sell these items for their listed price.
The "Trade Goods" table on page 157 of the Player's Handbook details examples of trade goods.