Sanford, Fla., police officer Timothy Smith holds up the gun that was used to kill Trayvon Martin during testimony at George Zimmerman’s 2013 trial. (Joe Burbank/AP)

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 stratum of this trade is “murderabilia,” the trafficking of sensational items from notorious crimes or killers. The online store Serial Killers Ink, for example, offers 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 such movies as “Psycho” and “The Silence of the Lambs.”

This is the market that George Zimmerman is trying to tap into by selling the semiautomatic gun he used to kill Trayvon Martin in the 2012 Florida slaying that riveted the nation. Zimmerman was acquitted in a sensational trial. Now, with the gun returned to him, 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.

George Zimmerman, who shot and killed 17-year-old Trayvon Martin and was acquitted of all charges, is selling the gun he says was used in the 2012 shooting. This is far from Zimmerman's first controversy since his acquittal in 2013. (Jenny Starrs/The Washington Post)

But if the American past is any prologue, he’ll be able to sell it — although probably not for the bogus $65 million bids the weapon has drawn so far — because Americans have always been fascinated with murder. Not surprisingly, people collect and trade crime 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 quiets the trade. Gacy, killer of 33 boys and 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 me in a 2008 letter when I was researching the trade. Berkowitz, known as the “Son of Sam,” or the “.44 Caliber Killer” when he was terrorizing New York in 1976 and 1977, is 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 D.C. museums.

Nor does every off-color 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 Stypeck, the proprietor of the District’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 U.S. Holocaust Memorial Museum and the Library of Congress. He appraised Booth’s derringer for $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, Stypeck 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 $450,000.

So does Zimmerman’s gun have social “value”? Would he touch it?

Tricky question, Stypeck 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 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,” Stypeck 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 reportedly 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 cultural merit. The national question concerns its ultimate destination, and how it is remembered.