So, in my last post's comments, we got into a small discussion about this question:
Why do adventurers risk death in dungeons instead of robbing the merchants, who, demonstrably have the money to buy the loot off of the adventurers?
In my games, anyway, what holds them back is combination of personality constraints, law, armed force, and long-term economic interest. None of this applies to monsters - it's generally legal in my games to kill off dangerous critters and recover lost or stolen goods and keep them.
Personality constraints - not everyone is purely greed motivated, and robbing Balto instead of sacking the Proving Grounds of the Mad Overlord just isn't an option for them. Paladin-types won't rob the merchants. Neither will priests of the Good God. Maybe the Settites have a strong orderly streak reflected in your Code of Honor or Vow or (for D&D types) alignment. Maybe they are purely greed motivated but have a strong line of separation between "doing a dangerous but profitable job" and "robbing businesspeople in town."
Law - it's illegal to rob the merchants, and so you suffer the threat of legal repercussions. In any legal system, too, resisting imposition of the law is also illegal, so you just compound the charges if you don't cease and desist and take your maiming or branding for robbery like a man. Scoff at the law? Well, the law has . . .
Armed force - even a badass can get swamped with numbers, and the townsfolk will have numbers. They may even have experience dealing with adventurer-types. And if they live near monsters they might have some reserve firepower to deal with it (siege engines, Greek fire, magical one-shots). They may include retired or current adventurer-types. Local weapon instructors might enjoy jumping in on a fight. They might even hire Toshiro Mifune and six of his buddies to kill you next time you ride on by to rob them. Even if they can't beat you in a fight they can make you a pariah, which sucks even if you are wealthy. Spend it on what, basically, if you're an exile from society?
And if you survive this, expect a bounty on you. Maybe not a huge one, but it'll be there, and it won't help you any. The bounty hunters don't come after you when you're ready to kick ass, they come after you when you're sleeping off that hellacious fight with the demonic lich-troll or out of arrows after fighting the orcs. Think repo-men. They don't carjack you, they come drive it or tow it away while you're asleep. You do want to be able to sleep soundly, right?
Finally, Long-term economic interest - aka greed. You could rob the merchants, but then what? In the long run, stealing (legally) from monsters and selling the take to merchants is a stable enough way to make money. A one-shot grab that makes it impossible to grab more isn't a good long-term idea. Don't burn the furniture. A merchant might have 4,000 sp for the 40% resale value of your 10,000 sp jeweled box but he probably doesn't have 10K . . . you won't get much more than selling him the stuff. And you can keep coming back, if you don't rob him.
There are other reasons why robbing them might not even be worth it.
Merchants might lack cash on hand. Maybe you sell you 10,000 sp jeweled box to them for 4,000 sp, but that doesn't mean you get 4K all at once. Maybe it takes a couple days to scrape the cash together, or it takes a couple of merchants to put the cash together. Or a consortium buys the big items - that would make sense, and explain why most RPG rules (and sane GMs) play out one transaction and say "that's the best price you find in town."
Or the money is stored at some local guildhouse or bank or whatnot. Maybe they issues letters of credit on big sales ("take this note to Bleign the Banker, he'll give you the money.") So maybe it's not even worth robbing them.
Of course, there is the old reliable "everyone is a retired high level adventurer" solution. That makes all of the above irrelevant - you can't rob Balto because he'll be tougher than W E R D N A. But I prefer the PCs to be the Conans and Elrics of the world, not go to them to sell their trinkets.
Finally, if you want to play of game of thieves and looters, that's totally fine. But it does mean sacking dungeons is probably not where your interest lies. If you really want to plunder the Ancient Tomb of Evil & Gold, it's best to not shit where you eat by killing the merchants.