How is this a viable business

Ah, Dave's slop shop, where you can exchange loot for rations. He's got a pot of bubbling liquid, about the same color as the swamp itself.

I hope it’s pea soup!
- Spoiler: the whole swamp is pea soup, the pot is not.
- It's A Cauldron of Endless Bog Water. The merchant lost it a while back and came here to retrieve it after tales spread of a new swamp popping up out of nowhere.

Good time for a crusty witch that scams the players and tries to harm them when they leave

Or...
Someone who drinks Baileys out of a shoe, goes to a club where people wee on each other and has a mangina. He wants you to stay. Forever.
Or she employs the old 'secretly give them a gold eating coin' trick. Momentary gain for eternal torment.

I bet someone has compiled a list of common video game tropes people try to bring into tabletop games. "Expecting a merchant to buy the blood soaked armor without questions" would definitely be on there.

Thankfully in PF2E, PC's only get half of most item's value when selling to merchants RAW. I like to use this as an explanation, especially to new players
- What is half the value of "a leather jerkin, soaked and blood and gut-juice, with two arrow sized holes and one axe sized hole"? Zero? Or perhaps a negative value because having it makes the owner look guilty of murder?
  I think I had a dm once say like "the armor didn't work for the last bloke, why would I want it?"
“Sleeping to regain hit points, multiple times a day, even when in a high speed chase” is up there for me.
- If I ever play D&D again, I think I'm going to spend a lot of time in session 0 getting on the same page about if we're playing D&D as a resource management game, or as a wacky hijinks game. So many people want to play the latter, but expect a long rest to be as easy as it is in video games.
  "I'm out of spells! Can we long rest?"
  "You're out of spells after casting nearly every single round of combat in the past 3 fights, and you want to chill out for 8 to 16 hours here, in the court of the evil duke's castle, while he's working on a ritual to summon a demon lord in a few hours?"
  I mean, I kind of get it. The game is set up so you have all these cool toys. Of course you want to use them. That conflict is why I dislike per-day resources.

That's when you reward your greedy players with a hag and a booth full of cursed items.

“Welcome to Swamp Swap! We buy and sell anything that floats.”

Lets see if this rusty armor floats then.

You can always encounter a traveling salesman. He will buy your stuff for cheap and sell you useful overpriced trinkets and exotic snacks.

Shrek needs armaments for his latest excursion

Doing business with adventures is just that lucrative.

Except it has three coins and a mushroom