From serial killer John Wayne Gacy's clown art to the Volkswagen in which Ted Bundy killed some of his victims, Americans have long been fascinated with the macabre aspects of sensational killings and the people who commit them.
The low strata of this trade is "murderabilia," the trafficking in sensational items from notorious crimes or killers.
The online store Serial Killers Ink, for example, is currently offering letters and artwork from both Charles Manson and the BTK Killer, Dennis Rader, as well as the confession signed by Ed Gein, the man whose skin-peeling killings inspired everything from Psycho to The Silence of the Lambs.
This is the market George Zimmerman is trying to tap into by selling the semiautomatic he used to kill Trayvon Martin in the 2012 Florida slaying that riveted the US.
Zimmerman was acquitted in a sensational trial, the gun is his property and, legally speaking, he's free to sell it as he would any other personal item.
Still, two gun sites have dropped the weapon from auction after public outrage.
But if the American past is any prologue, he'll be able to sell it - though probably not for the bogus US$65 million bids the weapon has so far drawn - because Americans have always been fascinated with murder.
Not surprisingly, people collect and trade in souvenirs, as they do with baseball cards or campaign buttons.
This market streams along regardless of the macabre taste involved. Legislatures try to ban criminals from selling stuff from prison, but it never slows the trade.
Gacy, killer of 33 young men in the Chicago area, worked with a seller in Louisiana to get around Illinois laws that sought to block the sale of his prison artwork.
"It's all-American style entertainment," David Berkowitz wrote to me in a 2008 letter when I was researching the trade.
Berkowitz, known as the "Son of Sam," or the ".44 Calibre Killer" when he was terrorising New York in 1976 and 1977, is now serving a life sentence.
He decries the murderabilia trade, those who try to sell his letters for profit, and the public fascination with homicidal violence - but he also knows it isn't going anywhere.
The flip side of the trade, though, is when a grisly item has historical significance and is going to be housed in some sort of educational setting. Then it really is worth something, in an aboveboard manner.
For example, the derringer that John Wilkes Booth used to assassinate President Abraham Lincoln, as well as the bullet extracted from Lincoln's brain, are both in DC museums.
Nor does every off-colour motive end badly.
The Mississippi judge in the original Medgar Evers trial took home the rifle that killed the civil rights legend after two hung juries in the early 1960s. He wanted it for a souvenir. Three decades later, it was rediscovered and used to help convict the killer, Byron de la Beckwith.
"Absolutely essential" to the case, says Jerry Mitchell, the Clarion-Ledger reporter whose stories led to the renewed prosecution and conviction of Beckwith.
Nobody in America is better versed in the ethics of dealing in these sorts of things than Allan Stypek, the proprietor of DC's Second Story Books. When he's not dealing with first editions of Faulkner, he's a professional and forensic appraiser of rare and historical items. He's done work for the Smithsonian Institution, the US Holocaust Memorial Museum and the Library of Congress. He appraised Booth's derringer for US$5 million.
When Robert White, the Maryland collector who had more than 100,000 items of John F. Kennedy memorabilia - from the significant to the blood-stained - died in 2003, Stypek helped with the sale of the controversial estate. In that sale, the flags that were on the presidential limousine when Kennedy was assassinated sold for US$450,000.
So does the Zimmerman's gun have social "value"? Would he touch it?
Tricky question, Stypek says.
The ethical question lies in the intent of the seller and buyer, he says. If a client wanted him to appraise a notorious weapon for insurance purposes, to be housed in a museum that showcases the horrors of the crime - like the derringer - that's fine. But for him to assess the same item for sale in an open market, that could mean profiting from an immoral object. And that's untenable, he says.
"That would be like asking me to appraise the last chalice Hitler drank from," he says. "You're in a whole other realm."
This slippery idea, he says, is "reverse historical merit" - meaning that an item's moral worth today directly contradicts the item's original intent. For example, the Nazis used Holocaust-related objects to try to destroy European Jews. Today, such Holocaust-related items are housed in museums to instead show the savagery of the Nazis.
That would be like asking me to appraise the last chalice Hitler drank from. You're in a whole other realm
That doesn't mean there isn't a parallel dark market in Nazi artifacts by racists and anti-Semites, nor does it mean that relatives of victims always have the highest interests in mind.
"I had a guy call with his mother's shirt when she was taken into Auschwitz," Stypek says. "I'm thinking this is of reverse historical merit. And he says, 'No, I want to know what I can sell it for.' "
One of the nation's most influential examples of the use of a homicidal "artifact" was, in fact, staged by the victim's mother.
In 1955, Emmett Till, a 14-year-old Chicago teen, was visiting relatives in tiny Money, Miss. He whistled at a white woman on a dare. The woman's husband and his half-brother kidnapped Till from his family's house a few nights later, tortured him, shot him in the head and dumped the corpse in a river.
When the body was sent home to Chicago for burial, his mother, Mamie, insisted on an open casket. Images of the child's mutilated face outraged the nation. It helped propel the civil rights movement and changed American history.
"Let the world see what has happened, because there is no way I could describe this," Mamie Till said. "And I needed somebody to help me tell what it was like."
Likewise, the killing of Trayvon Martin has become a modern cultural touchstone. And Zimmerman, like the killers of Till, went free.
It seems obvious that Zimmerman's gun is of immense cultural merit. The national question concerns its ultimate destination, and how it is remembered.