There is no sell button in UNDERDEEP and no player-to-player trading either: you cannot sell an item by hand, and you cannot hand an item to another player. Duplicate gear is sold for you automatically when you return from a run, and the money that produces is Coin, which buys avatar unlocks.
Underdeep selling items: there is no sell button
You cannot sell items by hand, because the Shop panel only offers skill upgrades. That is why the question comes up so often: there is no inventory button that turns a spare sword into gold.
Duplicates convert to Coin on the way out
When you return from a dungeon run, any duplicate of an item you already own is sold automatically and the better copy is kept. This is what makes the eight Loot slots workable: a full run usually comes back with several copies of the same item, and they simply convert into Coin on the way out.
What you cannot do mid-run is free space yourself. There is no way to drop an item or convert it to gold while you are inside the dungeon, and a high-rarity item can be lost simply because your eight loot slots were already full.
Coin buys random avatar unlocks
Coin is the gold currency shown next to Orbs on the character panel, and it is what you spend in town. The Avatar tab uses Coin to unlock character portraits, and each unlocked avatar carries stat increases - which is the reason to collect them all. The unlock itself is drawn at random rather than bought by choice, so the pull is random even though the reward is not.
No player-to-player trading
There is no trading of items between players in the released game: no selling an item to another player, no auction house, no item marketplace, and friends do not exchange gear. The only thing players exchange is gifts, which is where the friend-point currency comes from - and what that currency is worth is covered in the friend points guide.
'Route' means an auto path, not a trade run
The word route means something different here. A route is a path through a dungeon that you build out of map markers so your character repeats it on its own, and the developers describe three kinds of route explicitly: routes focused on uncovering unexplored tiles, routes that prioritise efficient character growth, and high-risk treasure-hunting routes that ignore danger.
The value of a route is measured in tiles per run against your Vigor budget and how many runs your Food and loop tickets allow. Because the dungeon layouts are fixed, a good route can be repeated and improved indefinitely. How to build one is in the route settings guide.