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

I hope it’s pea soup!

Or... Someone who drinks Baileys out of a slipper and has a mangina who wants you to stay. Forever.

Except it has three coins and a mushroom

Shrek needs armaments for his latest excursion

Lets see if this rusty armor floats then.

Spoiler: the whole swamp is pea soup, the pot is not.

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

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.

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.

What’s the flow rate of that cauldron if emptied or placed upside down? Based on your answer could become a quest or quest item in a campaign

[me trying to sell stuff in my yard] [I live in a swamp]

As always, it works at the Speed of Plot. Which somehow becomes much slower the second players come up with a use for an infinite source of unclean water.

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

Secondary Sign reads "change my mind".

Even the man is pea soup! And his name? Abraham Lincoln.

Or she employs the old 'secretly give them a gold eating coin' trick. Momentary gain for eternal torment.

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?"

I had to explain to one of my players once why no merchant was going to buy the dirty loincloths he was undressing dead goblins of; really didn't think that needed explaining

You misspelled it but it is.