"They don't have any money, and if they can help me make money, then it's only right to give some back."
Mr Voller has been slammed by family members of the killers' victims for profiting from their murders.
But the pieces and stories behind them are all just a part of modern history, according to him.
The creepy depictions of murder scenes, horror movie villains and violence obtained by news.com.au provide a disturbing insight into the minds of some of the world's most sadistic killers.
ANDRE CRAWFORD
Convicted serial killer and rapist, necrophiliac
Andre Crawford was an American convicted serial killer who killed 11 women between 1993 to 1999. He would smoke crack after killing them and later had sex with their corpses. In October 2016, he drew a picture from prison of a terrified woman with blood pouring from her face. He titled the image, First Kill.
"The 'First Kill' drawing is one of our favourite pieces by Andre," Mr Holler said.
"One can only assume that the drawing was his recollection of his first victim shortly before he killed her. This piece was on our website for all but 15 minutes before it sold. We knew it would sell fast as this is a prime example of the type pieces that are in demand.
"We obtained all of our Crawford pieces through a third party."
Crawford died earlier this year.
HADDEN CLARK 'The cross dressing cannibal serial killer'
Convicted serial killer and cannibal
Hadden Clark, 65, is currently serving two 30-year sentences for the 1992 murder of 23-year-old Laura Houghteling and the 1986 slaying of six-year-old Michele Dorr in the US.
Clark reportedly spoke like a woman and an infant when he was questioned by police over the killings.
Detectives said Clark's female personality said he had buried "them" in New Jersey, without saying whom or where.
The killer reportedly told a cellmate that he murdered at least a dozen people and that his female alter ego was responsible for those deaths.
In an undated handwritten note on a drawing Clark prepared for Mr Holler, he referred to himself as 'Kristen'.
"Dear Eric, Here is a picture of me when I was a little girl. When I was little I love (sic) to go to Sunday school so much that people started calling me Kristen because I was a Christian and I love Jesus. Love Kristen," Clark wrote.
Mr Holler said the "Hadden Clark Kristen drawing was another piece that sold very quickly".
But he wasn't sure if the killer had really depicted his alter ego in the drawing.
"The drawing also looks just like the infamous photograph of his (child) victim Michelle Dorr," Mr Holler said.
"We can't help but to notice the eerie similarity in the way he drew 'Kristen' to that of his victim Michelle Dorr."
Clark's penchant for female clothing was said to have started when his mother clothed him in dresses and ruffles as a baby because she wanted a daughter.
"If you follow the story of Hadden Clark you know that his mother, when he was a child, would dress him as a little girl and call him Kristen," Mr Holler said.
ROBERT BARDO
Convicted killer
At 21-years-old, actor-model Rebecca Schaeffer was shot dead at the door of her Los Angeles apartment by obsessed fan Robert Bardo on July 19, 1989.
The American killer, who had written Ms Schaeffer letters and tried to meet her at a studio where she worked, tracked the My Sister Sam actor down after paying a private investigator to obtain her home address from state motor vehicle records.
In prison, he drew a picture of horror movie villain 'Chucky' holding a knife in each hand. It was later sold via Serial Killer Ink.
Bardo remains in prison, serving a life sentence without possibility of parole.
JOHN WAYNE GACY, 'Pogo the clown'
Prolific convicted serial killer
He was the friendly neighbour who dressed as 'Pogo the Clown' at parties and visited sick kids in hospitals. But behind the facade of his clown mask, John Wayne Gacy hid a shocking secret. Buried under the floor of his suburban Chicago home were the bodies of 28 boys and young men. Another body was entombed in his driveway, while four more were dumped in a nearby river.
Gacy murdered at least 33 people before he was imprisoned in the 1970s. While behind bars he painted several pictures including one of 'Pogo the clown' and another of a skull wearing a clown's hat.
Gacy was executed by lethal injection in 1994.
JOSEPH DRUCE
Convicted killer
Joseph Druce is serving two life sentences for the separate murders of a gay bus driver and a paedophile priest.
Druce was given a life sentence without parole for killing convicted paedophile priest John Geoghan in 2003 while the two were in a correction facility.
He was already serving a life sentence for beating and killing a gay bus driver in 1988.
One of the pictures he has drawn shows a decapitated man lying bloodied on the floor of a prison cell as a man stands by holding his head is accompanied by the tagline: 'Stop the EVIL! Leave the children alone now' .
"The Druce 'Stop the Evil' drawing is a classic," Mr Holler said.
"It's no secret that many view and hold Druce as a modern day hero and he quite seems to revel in it.
"We love the tagline 'Leave the Children Alone' and rightfully so."
RODERICK FERRELL
Convicted killer
Four teenagers whom police alleged had slit their wrists and drank each other's blood were accused of bludgeoning to death a Florida couple in 1996.
Roderick Ferrell - the 16 year old ex boyfriend of the victims' daughter, Heather - was charged with murder while three others were charged with assisting the murder.
A police spokesperson said it "appeared they got caught up in a fantasy vampire game that mushroomed into a role-playing cult".
In 2013, Ferrell sketched a picture of the horror movie character Michael Myers from Halloween before it fell into the hands of Mr Holler and was sold on his site.
RICHARD RAMIREZ, 'The Night Stalker'
Convicted serial killer and rapist
Richard Ramirez was a devil worshipper also known as the satanic 'Night Stalker'.
Ramirez was convicted in 1989 and sentenced to death for 13 murders, five attempted murders, 11 sexual assaults and 14 burglaries.
Mr Holler later obtained a drawing by the killer that depicted a pair of wicked eyes above a bloodied knife.
Ramirez's monstrous rampage from Los Angeles to San Francisco paralysed the state in the mid 1980s. Among his victims was Jennie Vincow. Ramirez climbed through Ms Vincow's home window and slit her throat, nearly decapitating her. He then sexually assaulted her corpse.
Ramirez died of cancer in 2013 before he could be executed.
EUGENE McWATTERS, The Salerno Strangler
Convicted serial killer
Eugene McWatters was convicted of the 2004 slayings of three women, Jackie Bradley, Christal Wiggins and Carrie Ann Caughey, in Port Salerno, Florida. The women, whose ages ranged from 18 to 43, were found raped and strangled within 10 weeks of one another.
His drawing of skeletons in black capes hovering above a cemetery was commissioned by Mr Holler who sold the artwork via Serial Killer Ink.
McWatters, dubbed the 'Salerno Strangler', pleaded no contest to three counts of first-degree murder just at his 2015 retrial. He was sentenced to life in prison without parole.
CHANE VAN HEERDEN
Convicted killer
Chané Van Heerden was convicted of an occult murder in the South African town of Welkom in 2011.
Her drawings in prison reveal that the killer still has disturbingly ghoulish interests.
"Chane van Heerden sent us a beautiful drawing of a female zombie who is enjoying the act of ripping her entrails out," Mr Holler told news.com.au.
Van Heerden and her accomplice Maartens van der Merwe killed, dismembered their victim. After the killing, Van Heerden used skinning techniques she had learnt while hunting with her stepfather as a young girl. The couple then buried various parts of the remains in a local cemetery as well as in their backyard garden. According to the Independent Online, victim Van Eck was decapitated, his right arm cut off, and both his legs amputated at the knees. Mr Eck's eyes and ears were found in the couple's fridge.
