Joss Whedon, whose newest movie Avengers: Age of Ultron hit theaters recently, has one particular economic (and ethical) preoccupation that reappears in his works: What should be for sale?

Whedon’s particular preoccupation becomes apparent in season 6 of Buffy the Vampire Slayer. After Buffy’s mother dies in season 5, Buffy is left alone with financial responsibility for herself and her younger sister. She has a checkered high school education, a spotty year at UC Sunnydale, and no practical job experience. She also has leaking pipes, a roof that needs to be replaced, and a house that is losing value because of its proximity to the Hellmouth. Money is clearly a problem.

It is the ever-practical Anya (the former demon transformed into a human and an enthusiastic proponent of capitalism) who has the solution.

Anya: Um ... i-i-if you wanna pay every bill here, and every bill coming, and ... have enough to start a nice college fund for Dawn? Start charging.

Buffy: For what?

Anya: Slaying vampires! Well, you’re providing a valuable service to the whole community. I say cash in. (“Flooded,” season 6, episode 4)

Though Buffy and the other humans dismiss the idea almost as quickly as it is raised, Anya’s suggestion that Buffy start charging for slaying vampires and demons is — ahem — resurrected later in the episode. When a demon attacks the bank where Buffy is applying for a loan, she dispatches him quickly, then turns to the loan officer to remark, “I’m not saying I’m charging you for saving your life or anything, but ... let’s talk rates.” This doesn’t work, but the series now has opened the question: Why doesn’t Buffy get paid for slaying? Why isn’t it okay for her to charge?

Anya isn’t the only one who is perplexed. Fan discussion boards (remember those?) during the run of the show were rife with topics titled “Why doesn’t the Council pay Buffy?” “Plot hole: Buffy not getting paid to be a slayer,” and “Seriously, how ridiculous is it that no one financially helped Buffy?”

Fan fiction writers often sought to remedy the injustice by, for example, promoting Giles to head of the Watchers Council and giving Buffy a salary for her work. Ms Summers Goes to Washington by the author “Beer Good” even crossed Buffy the Vampire Slayer with The West Wing and sent Buffy to Washington and the EU to demand financial assistance. Popular blog The Toast posted a parody version of Buffy as written by Ayn Rand that seems a lot closer to the concerns of the show than the parodists probably intended:

Giles: In every generation there is a Slayer. She is the Chosen One. She alone will stand against the forces of darkness —

Buffy: What does it pay?

Giles: What do you mean?

…

Buffy: Surely this kind of specialized labor merits compensation, if my skills are so highly valued on the free market.

The characters on Buffy the Vampire Slayer might think that slayage doesn’t deserve compensation. Their fan base isn’t so sure.

And I don’t think Whedon is so sure, either. The question just keeps coming up in his work. In Dr. Horrible’s Sing-Along Blog, the superheroic Captain Hammer is referred to as a “corporate tool” in a quick glance at the question of selling heroism. And in the episode “I Fall to Pieces” from season 1 of Angel, the vampire Angel comes to terms with the fact that he will have to charge for his detective and security services. His coworkers remind him that he’s been busily saving lives and getting nothing in return. Angel notes, “I don’t feel comfortable asking for money.”

He is urged to think of charging as a way of creating emotional distance from his clients. “People get attached to a mysterious savior, and can you blame them? But as long as you’re just a man who’s doing a job, and getting paid, they can feel like they’ve paid their debt to you and they can move on — independent like.”

And by the end of the episode — during which he defeats a mad sorcerer/doctor who had been magically stalking a former lover — Angel has learned both the value of not getting overly close and the value of his services. Sometimes it appears to be okay to charge for being heroic.

These questions about what should be for sale are even more complicated in Whedon’s next big TV series: Firefly. The outlaw crewmembers of the spaceship Serenity are known for taking whatever jobs they can get and not asking too many questions. But the series itself does nothing but ask those questions. From discussions about Inara Serra’s job as a “companion” who sells both sexual and emotional services, to debates among the crew about whether the fugitives River and Simon Tam should be turned in to the government for ransom, Whedon’s interest in the question of what should be for sale is front and center.

In the early episode “The Train Job,” the crew is hired by Adelai Niska to remove some unspecified cargo from a train and deliver it to him. The job goes awry in a number of ways, not the least of which is that the crewmembers discover that the cargo they have been hired to steal is medicine that colonists require for survival. Aware that no one is going to help the settlers if he doesn’t, Captain Mal resolves to return the advance deposit Niska paid him, return the medicine to the settlers, and walk away. Niska objects; violence ensues. The settlers get their medicine, Niska gets his money, and Mal gets an implacable enemy.

We are left, as viewers, feeling that Mal has done the just thing here. It would be easy to conclude from this that we don’t believe in selling medicine. But I think it’s more that we don’t believe in selling medicine that already belongs to other people. It’s not the market behavior that’s the problem. It’s the violation of property rights.

Some evidence for this subtlety appears in the following episode, “Bushwhacked.” Mal and the crew encounter an abandoned ship filled with a fortune in colonists’ supplies. They take the supplies back to Serenity and then undergo some harrowing encounters with government agents and with savage Reavers. At the end of the episode, the supplies are confiscated by the government. Mal’s crewmember Jayne is appalled. “You save his gorram life. He still takes the cargo.”

Mal responds, “He had to. Couldn’t let us profit. Wouldn’t be civilized.”

Like Mal and Jayne, we feel that the government committed a wrong by confiscating the crew’s windfall. This was property that — through tragedy — belonged to no one. And while the supplies they found were doubtless as life-sustaining as the medicine that was such an issue in “The Train Job,” because we have no sense that another owner has a prior claim over those supplies, we feel that the government’s confiscation of them is just as much a violation as Niska’s theft of medicine.

More recent works in the Whedonverse have continued to explore the question of what should be for sale. Dollhouse centers on a corporation that temporarily programs individuals with skills and personalities and then rents them out to customers. The world of the series, in other words, is based on that question.

Even Agents of SHIELD — where Whedon is somewhat constrained by the necessity of playing within the bounds of the Marvel universe — has an episode where the central problem of the plot revolves around the question. In “The Girl in the Flower Dress,” new SHIELD trainee Skye helps her friend and lover Miles to evade a SHIELD team when they arrive to apprehend him for hacking into SHIELD’s database. Skye assumes he has done so as a prank. When she discovers that he sold the information he obtained, she is horrified, particularly because the information is likely to lead to the death of at least one human.

Even Whedon’s philosophical sci-fi horror movie Cabin in the Woods has something to add to our thinking about the question of what should be for sale. Whedon has explained that his intent with Cabin in the Woods was to challenge the current vogue for torture porn in horror movies that basically depict, as he put it, “kidnap, torture, and execution, in that order.” Horror movies, he argues, are now simply “trying to upset you, as opposed to trying to scare you.” The starting point for the movie was, in other words, what should be for sale in horror movies? Graphic violence against fictional characters or an adrenaline rush for the audience?

With Whedon’s newest entry in the Avengers series just out — with what appears to be an epic battle of dozens of superheroes and villains set to unfold — has Whedon found time to think about whether, how, and why they might be getting paid?